Slab for rail system

Abstract

A slab 100 for a rail system formed from a monolithic piece of fibre reinforced concrete having a height of no more than 15 cm and which comprises a pair of longitudinal grooves 111, 112. The grooves

Description

FIELD OF THE INVENTION The present invention relates to the field of railway infrastructure. BACKGROUND OF THE INVENTION There is a long tradition of railway infrastructure that provides rails along which a rail-based vehicle, such as a tram or train, can be propelled. In traditional railway infrastructure, rails are mounted upon railway sleepers or ties, which lie perpendicular to the direction of the track. Each railway sleeper provides two fixed locations to which the rail can be secured. The railways sleepers themselves are usually lain upon track ballast, typically formed of crushed stone (e.g. gravel), so that a load carried by the railway sleepers (e.g. of the tram / train) is distributed into the track ballast. Another type of railway infrastructure is known as a “ballastless” or “slabtrack”, in which multiple railways sleepers are mounted on, or integrated in, a single (concrete) slab. Thus, a slab is able to couple to each rail at two or more different locations. This approach avoids the need for ballast, and provides advantages of improved performance capacity, reduction in maintenance cost / complexity and improved lifespan. Conventional slabs for railway infrastructure (sometimes called “precast slabs”) have fixed positions to which the rails can be secured. Usually, these fixed positions are arranged to simulate a linear arrangement of railway sleepers, e.g. comprise two, parallel groups of linearly arranged fixed positions. Existing slabs are usually formed from reinforced or pre-stressed concrete, and generally have a minimum thickness of around 17 cm. Moreover, a slab is typically around 5 m long and 2.20 m wide resulting in a high weight of around 5 tons. This makes it necessary to utilize heavy-load handling systems making the handling and transportation of these slabs relatively expensive. There is an ongoing desire to improve slabs for railway infrastructure, and in particular, to make slabs more suitable for use in an urban environment. SUMMARY OF THE INVENTION The invention is defined by the claims. According to examples in accordance with an aspect of the invention, there is provided a slab for a rail system, the slab comprising: a pair of longitudinal grooves provided at a first side of the slab, wherein each longitudinal groove is configured to receive and support a respective rail for directly engaging with a wheel of a rail-based vehicle, wherein the slab is formed from a monolithic piece of fiber reinforced concrete and has a height of no more than 15 cm. A slab, sometimes called a “track slab” or “precast slab”, is a well-known term in the rail industry to refer to a panel or sheet of material that can act to distribute load and provide track stability. The slab is generally shaped to have a length and width much greater than its height / depth, and may be formed in a cuboidal shape, a trapezoidal prism shape and so on. The present disclosure recognizes that it is possible to directly embed a pair of rails in a thin slab to enable a (light) rail system with a low overall thickness that is suitable for installation in urban environments in which existing buried infrastructure is provided close to a road surface, such as bridges. More particularly, it has been recognized that a slab formed of fiber reinforced concrete has sufficient flexural strength to support a rail-based vehicle on rails embedded in the slab even at an overall thicknesses of 15 cm or less. In some examples, a distance between a base of each longitudinal groove and a surface on a second side of the slab, opposite to the first side, is no more than 10 cm. In other words, a height of the slab directly beneath a rail received in each longitudinal groove is no more than 10 cm. The present disclosure recognizes that a slab formed from fiber reinforced concrete is able to provide sufficient flexural strength to support a rail-based vehicle on the embedded in the slab even when a portion of the slab that directly supports each rail has a thickness of 10 cm or less. In some examples, the distance between the base of each longitudinal groove and the surface on the second side of the slab is between 4 cm and 6 cm. In some examples, the slab is formed from ultra-high performance fiber reinforced concrete. It has been recognized that ultra-high performance fiber reinforced concrete provides still greater strength for the same thickness, allowing the desired strength of the slab for supporting a rail-based vehicle to be achieved at even lower thicknesses. In some examples, each longitudinal groove is formed by a respective pair of longitudinal protrusions extending from the first side of the slab. In this way, portions of each slab on either side of the pair of rails and between the pair of rails may be covered with a pavement or surface course, providing a suitable surface for road-based vehicles, while keeping the top of the rails exposed. Further, the reduction in fiber reinforced concrete achieved by this design reduces a cost of the rail system. In some examples, a thickness of each longitudinal protrusion, in a direction perpendicular to both a direction in which the pair of longitudinal grooves extend and the height of the slab, is no more than 15 cm. For instance, the thickness of each longitudinal protrusion may be between 5 and 10 cm. In some examples, a respective portion of the slab bordering each side of each longitudinal groove has a textured surface. This provides skid resistance. Where the grooves are formed by protrusions extending from the slab, the textured portions may be the upper surfaces of the protrusions. In some examples, a height of the slab is no greater than 12 cm. This further reduces the excavation required to install the slab, and allows the slab to be installed without disruption to existing infrastructure in locations in which existing infrastructure is even closer to a road surface. There is also provided a slab system for a rail system, the slab system comprising: the slab described above; and for each longitudinal groove, a respective rail positioned in the longitudinal groove for directly engaging with a wheel of a rail-based vehicle. Each rail may be a block rail (i.e. a rail without a web between the rail head and foot). Block rails have a lower height than traditional grooved rails, allowing the overall height of the slab system to be further reduced. In some examples, each rail is embedded in an elastomeric material provided in the respective longitudinal groove. This provides continuous support and vibration damping along the length of the rail, reducing wear on the rails. According to examples in accordance with an aspect of the invention, there is provided a slab arrangement for a rail system, the slab system comprising: a plurality of slabs, each slab being a slab as described above, wherein the pairs of longitudinal grooves of adjacent slabs are aligned with one another to form a pair of longitudinal channels. In some examples, the slab arrangement further comprises, for each longitudinal channel, a respective set of one or more rails positioned in the longitudinal channel. In some examples, for each longitudinal channel, each rail is contained entirely within the longitudinal channel such that a height of the slab arrangement is no more than 15 cm. In some examples, for each longitudinal channel, each rail is embedded in an elastomeric material provided in the respective longitudinal channel. The elastomeric material may be a polyurethane elastomeric. Polyurethane elastomeric materials, such as Corkelast®, provide strong adherence between the rail and slab. In some examples, adjacent slabs are coupled by a clamp. This increases a stiffness of the rail system at boundaries between adjacent slabs. The clamp may, for example, be an elongate U-shaped clamp. In some examples, the clamp may be made of steel. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter. BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example only, to the accompanying drawings, in which: Figure 1 illustrates a slab for a rail system, according to an embodiment of the invention; Figure 2 illustrates a side view of the slab of Figure 1; Figure 3 illustrates a slab system for a rail system, according to an embodiment of the invention; Figure 4 illustrates a slab arrangement for a rail system, according to an embodiment of the invention; and Figure 5 illustrates a slab system for a rail system, according to another embodiment of the invention. DETAILED DESCRIPTION OF THE EMBODIMENTS The invention will be described with reference to the Figures. It should be understood that the detailed description and specific examples, while indicating exemplary embodiments of the apparatus, systems and methods, are intended for purposes of illustration only and are not intended to limit the scope of the invention. These and other features, aspects, and advantages of the apparatus, systems and methods of the present invention will become better understood from the following description, appended claims, and accompanying drawings. It should be understood that the Figures are merely schematic and are not drawn to scale. It should also be understood that the same reference numerals are used throughout the Figures to indicate the same or similar parts. The invention provides a slab for a rail system. The slab is formed from a monolithic piece of fiber reinforced concrete having a height of no more than 15 cm, and comprises a pair of longitudinal grooves. The longitudinal grooves are provided at a same side of the slab, and are each configured to receive and support a respective rail. Illustrative embodiments may, for example, be employed in light rail systems, such as tram systems. Figure 1 illustrates a slab 100 for a rail system, according to an embodiment of the invention. The slab is formed from a monolithic piece of fiber reinforced concrete and comprises a pair of longitudinal grooves 111, 112 (i.e. grooves extending along the length of the slab, from end to end) provided at a first side 100a of the slab. The term “slab” has a well-established meaning in the railway infrastructure field to refer to a generally rigid piece of material upon which rails are mounted. In particular, a slab is generally able to couple to a single rail at two or more different locations (e.g. compared to a sleeper, which connects to a single rail at a single location only). Thus, when supporting two rails, a slab is able to be coupled to each rail at two or more different locations (for each rail). The slab 100 according to the present disclosure is configured to enable a pair of rails to be mounted within the pair of longitudinal grooves 111, 112. In other words, each longitudinal groove is configured to receive and support a respective rail. As used herein, the term “rail” refers to a rail configured to directly engage with a wheel of a rail-based vehicle. The mounting of a rail within each longitudinal groove is described in more detail below. A slab is shaped to have a length (in the direction z of rails to be positioned on the slab), a width (generally in a direction y perpendicular to the direction z) and a height / depth, which lies in the vertical direction x. The illustrated slab 100 is generally cuboidal, although other possible 3D shapes are plausible, such as trapezoidal prisms, chevron prisms, and so on. In Figure 1, the slab and the longitudinal grooves 111, 112 are straight; however, in some examples, the slab and / or the longitudinal grooves may be curved, as described in more detail below. As previously mentioned, the slab 100 is formed from a monolithic piece of fiber reinforced concrete. Fiber reinforced concrete improves the elastic and fatigue resistance of the slab, compared to conventional concrete (which is used as standard in conventional slabs for rail systems). The use of fiber reinforced concrete thus allows the slab to have a low overall height (compared with conventional slabs), while still being able to support a rail-based vehicle thereon. Moreover, the use of fiber reinforced concrete allows the slab to be cut on-site without significantly affecting the structural integrity of the slab and means that the slab is unlikely to crack or break while being moved, thus facilitating installation of the slab. The ability to cut the slab without significantly affecting the structural integrity of the slab further allows infrastructure beneath the slab (e.g. pipes and / or cables) to be accessed without damaging the slab. In some examples, the slab is formed from ultra-high performance fiber reinforced concrete (UHPFRC). UHPFRC has a standardized ruleset, e.g. as set out in the French standard NF P 18-710, “National Addition to Eurocode 2 — Design of Concrete Structures: Specific Rules for Ultra-High Performance Fiber-Reinforced Concrete (UHPFRC)”. Another example standard for UHPFRC is set out by the Swiss Standard SIA 2052 UHPFRC. Another mechanism for defining an ultrahigh performance fiber reinforced concrete is to define a UHPFRC as a fiber reinforced concrete that meets an ultrahigh performance concrete (UHPC) standard, such as the ASTM Cl856 testing standard. The skilled person would therefore readily understand the meaning and scope of the term UHPFRC. The use of UHPFRC enables particularly thin slabs to be produced while still providing the flexural strength to support a rail-based vehicle on the rails embedded in the slab. A slab formed from UHPFRC is therefore advantageous in sites in which existing infrastructure is particularly close to the surface. The longitudinal grooves 121, 122 of the slab 100 are positioned to generally align with an intended travel direction for a rail-based vehicle (that is be supported by the slab 100). Thus, the length of the slab lies generally in the intended forward and rearward travel direction for the rail based vehicle. In Figure 1, each longitudinal groove is formed by a respective pair of longitudinal protrusions 121, 122 extending from the first side 100a of the slab. In other words, each longitudinal groove is formed between a first longitudinal protrusion 121 and a second longitudinal protrusion 122. In some examples, an uppermost surface of each longitudinal protrusion (i.e. a surface furthest from the base of each longitudinal groove in the direction x) may have a textured surface 125 to provide skid resistance. The textured surface may have a skid resistance value of at least 45 PSRV (polished skid resistance value), as required for low-risk sites by the Design Manual for Roads and Bridges (DMRB) Standard CD 534. In some examples, the textured surface may have a skid resistance value of at least 60 PSRV, as required for high-risk sites by the DMRB Standard CD 534. This can be achieved through appropriate texturing of the textured surface, by embedding material into the slab and / or providing another material on top of the slab. In Figure 1, the textured surface 125 is in the form of a diamond grid pattern; however, as the skilled person will readily appreciate, the textured surface may take any form that provides the desired skid resistance. In some examples, the textured surface may be formed from a separate material to the slab 100. In some examples, rather than each longitudinal grooves being formed by a pair of longitudinal protrusions, the spacing between the pair of longitudinal grooves (excepting any through-holes or cut-outs in the slab) may be filled with fiber reinforced concrete. In other words, the height of the slab at any point other than a longitudinal groove may be equal to the overall height of the slab. Conceptually, this may be considered as equivalent to the slab having longitudinal protrusions in which the longitudinal protrusions closest to the edges of the slab each extend to the respective edge of the slab and / or the slab comprising a single protrusion having a width of the distance between the longitudinal grooves provided between the longitudinal grooves. A respective portion of each of the slab bordering each side of each longitudinal groove may have a textured surface, as described above. The shape of the slab 100 (and, where present, the textured surface) may be achieved by cast molding the slab. For instance, the textured surface may be formed by texturizing the relevant portion of the casting tool used to mold the slab, or (where the textured surface is formed using a separate material to the slab) by placing a suitable material (e.g. stone chippings, such as granite chippings) in the casting tool before pouring the fiber reinforced concrete into the casting tool, so that the material becomes embedded in the fiber reinforced concrete to provide the textured surface. In some examples, the slab 100 may comprise one or more through-holes 130 (i.e. holes that extend from the first side 100a to a second side, opposite to the first side). These through-holes allow the slab to be lifted and moved, e.g. for the purposes of installation, removal and / or replacement of the slab. In particular, the through-holes may be threaded to allow rods / screws to pass therethrough in order to facilitate a slight raising of the slab (e.g. when it is positioned on the ground). This can allow, as will be later described, bedding material to be cast or poured beneath the slab. In some examples, bedding material may be poured beneath the slab by pouring the bedding material through the through-holes. The through-holes may be threaded by providing threaded inserts within the through-holes (e.g. inserts formed from a suitable polymer or metal). In some examples, the slab 100 may further comprise one of more cut-outs (provided in the region between the longitudinal groove and / or on either side of the longitudinal grooves) for enabling access to existing infrastructure (e.g. a cut-out for a manhole). The one or more cut-outs may, for example, be cut on-site during installation of the slab. In some examples, the slab 100 may comprise one or more water seepage holes 131 extending from the first side 100a of the slab to the second (opposite) side, to enable water or moisture drainage. In other words, the one or more water seepage holes allow water to drain therethrough, from the first (upper) side of the slab to the second side, thus reducing the collection of water on the upper surface of the slab. Each water seepage hole may, for example, have a diameter no greater than 25 mm. The one or more water seepage holes 131 may be particularly advantageous when the slab 100 is formed from UHPFRC, which tends to be impermeable or of very little permeance (compared to conventional concrete). The one or more water seepage holes are also advantageous when a surface course is provided on portions of the slab outside / between the pairs of longitudinal protrusions (as described in more detail below). The water seepage holes help avoid water stagnation at the interface between the slab and the surface course, and thereby improve a longevity of the slab. Figure 2 illustrates a side view of the slab 100. Figure 2 illustrates more clearly the dimensions of the slab. In particular, the slab has a height H (i.e. a thickness in the x direction) of no more than 15 cm, for instance a height between 9 and 15 cm. In some examples, the height of the slab may be no greater than 12 cm (e.g. between 9 and 12 cm). This low height (compared with conventional slabs) allows the slab to be installed at sites in which existing infrastructure is provided close to a road surface (e.g. on bridges), without unduly disturbing the existing infrastructure. From the foregoing, it will be apparent that, in some examples, the height of the slab is no less than 9 cm. This has been identified as defining sufficient flexural strength for a slab of fiber reinforced concrete to not bend or crack during movement or lifting. Ultra-high performance fiber reinforced concrete may, in practice, be thinner. For instance, a slab formed from UHPFRC may have heights as low as 6 cm while still providing sufficient flexural strength to support a rail-based vehicle. For instance, a slab formed from UHPFRC may have a height between 6 cm and 9 cm. The enhanced strength properties of UHPFRC allow for these reduced slab thicknesses compared to conventional fiber reinforced concrete, while still maintaining the necessary load-bearing capacity for rail applications. Preferably, a distance D between a base of each longitudinal groove and a surface on the second side 100b of the slab (opposite to the first side 100a), is no more than 10 cm, for instance between 4 cm and 10 cm (e.g. between 4 cm and 8 cm). In some examples, the distance D may be between 4 cm and 6 cm. A thickness T of each longitudinal protrusion 121, 122 in the y direction (perpendicular to the height and length of the slab) may, for example, be no more than 15 cm, for instance, between 5 and 15 cm (e.g. between 5 and 12 cm). In some examples, the thickness T may be between 5 and 10 cm, for instance, between 7 and 9 cm (e.g. 8 cm). This provides longitudinal protrusions each having a sufficient thickness (and therefore structural integrity) to support a rail within the longitudinal groove formed by the longitudinal protrusions, and also allows a sufficient width of textured surface (where present) to provide the desired skid resistance. As the skilled person will readily appreciate, the dimensions of each longitudinal groove 111, 112 may depend on dimensions of a rail to be accommodated within the longitudinal groove. For instance, the width W of each longitudinal groove (i.e. a distance between the longitudinal protrusions 121, 122 forming the groove) may at least 1 cm wider than a width of a rail to be mounted in the groove (in order to account for manufacturing tolerances and / or elastomeric material). In some examples, the width W of each longitudinal groove may be wide enough to accommodate a rail having a different curvature to the longitudinal groove, for instance no less than 2 cm wider and / or no more than 5 cm wider than a width of a rail to be mounted in the groove (e.g. 2.5-4.5 cm wider). For instance, a longitudinal groove for mounting a rail having a width of 156 mm may have a width W of 180-200 mm. In some examples, particularly for slabs intended for use in areas designed for mixed-vehicle traffic, the depth d of each longitudinal groove may be deep enough to allow each rail to be entirely contained with the respective longitudinal groove (i.e. such that the rail does not extend beyond the longitudinal protrusions forming the respective longitudinal groove). For instance, the depth d of each longitudinal groove may be at least 5 mm greater than a height of the rail to be mounted in the groove (in some examples, at least 15 mm greater than the height of the rail to be mounted in the groove, in order to permit provision of an elastomeric material between the base of the longitudinal groove and the rail). As the skilled person will readily appreciate, a distance between the longitudinal grooves 111, 112 (i.e. a distance between a first longitudinal groove 111 and a second longitudinal groove 112) will depend on a desired gauge of the rail system for which the slab 100 is intended. A distance X between an outer edge of each outer longitudinal protrusion (i.e. an outer edge of the longitudinal protrusion in each pair closest to an outer edge of the slab) and a closest outer edge of the slab may depend on installation requirements (for example, on the position of existing infrastructure). In some examples, the distance X may be zero (i.e. the outer edge of each outer longitudinal protrusion may be flush with a respective outer edge of the slab), in order to reduce a width of a trench to be milled for installing the slab. In other examples, the distance X may be at least 5 mm, in order to provide structural support to the outer longitudinal protrusions and, optionally, to enable a clamp to be mounted on a portion of the slab outside the longitudinal protrusions (as described in more detail below). The length (in the longitudinal direction z) of the slab 100 may depend on requirements of the site at which the slab is being installed, and on a curvature of the longitudinal grooves. For instance, a slab with straight longitudinal grooves may have a length of 6-8 m, which is short enough that the slab is relatively easy to transport and handle, but long enough to reduce the number of discontinuities in the track. Slabs with curved longitudinal grooves may be shorter, as the width of a curved longitudinal groove would need to increase with the length of the slab, resulting in the use of an inefficient amount of elastomeric material, where present (see below). For example, a slab with curved longitudinal grooves having a radius of curvature of 15-40 m may have a length of 2-4 m (e.g. a length of 3 m), while a slab with longitudinal grooves having a radius of curvature of 40-150 m may have a length of 4-6 mm (e.g. a length of 5 m). In some examples, the slab may be manufactured in a variety of lengths, in order to enable a desired length of track to be laid. Thus, the length of the slab is not necessarily fixed. Additionally or alternatively, the slab may be cut to a desired length on-site. Figure 3 illustrates a slab system 300 for a rail system, according to an embodiment of the invention. The slab system 300 comprises the slab 100, described above with reference to Figures 1 and 2, and a pair of rails 340. Each rail is positioned in a respective longitudinal groove 111, 112 of the slab, and is configured to directly engage with a wheel of a rail-based vehicle. Preferably, a top surface of each rail may be flush with an uppermost surface of the slab (e.g. with an uppermost surface of each longitudinal protrusion), in order to provide a level road surface for other vehicular traffic. It will be appreciated that a level surface reduces the risk of accidents and improves overall safety for all road users, and enables other vehicles to cross or travel alongside the rails without experiencing significant bumps or jolts. However, this is not essential. For instance, in dedicated rail corridors, it is not essential for the top surface of each rail to be flush with the uppermost surface of the slab (e.g., where mixed traffic is not a consideration). In some examples, the slab may be one of a plurality of slabs of a slab arrangement, as described in more detail below. In Figure 3, each rail 340 is a block rail, having a rail head similar to a traditional grooved rail (i.e. with a groove in the rail head for engaging with a wheel of a rail-based vehicle), but without a web structure separating the rail head from the foot of the rail. Block rails have a lower height than traditional grooved rails, allowing a depth of the longitudinal grooves, and therefore a lower overall height of the slab, compared with the use of traditional grooved rails. The rails may conform to the EN14811 standard. Each rail 340 may be embedded in an elastomeric material 350 provided in the respective longitudinal groove 111, 112 (e.g. the elastomeric material may fill any region of the longitudinal groove between the slab 100 and the rail). The use of an elastomeric material between each rail and the slab 100 provides continuous support along the length of each rail, absorbing shocks and damping vibrations, thus reducing wear on the rails as well as performing a noise reduction function. A top surface of each rail (i.e. a surface containing the groove in the rail head) is exposed (i.e. not coated in the elastomeric material). The elastomeric material also secures each rail within the respective longitudinal groove, and protects the rails from water ingress, thus reducing rusting. Any suitable elastomeric material 350 may be provided between the rail 330 and the slab 100. For instance, the elastomeric material may be a polyurethane elastomeric, such as Edilon)(Sedra Corkelast® or a Sika® Icosit® resin grout, which provides a strong adherence between the rail and the slab. Edilon)(Sedra Corkelast® is a rail fastening system comprising a polyurethane resin and a filler (e.g. cork). In some examples, the slab system 300 may further comprise a shock absorber 360 (e.g. a shock absorbing mat, strip or pad) provided between each rail and the base of the longitudinal groove in which the rail is received. The shock absorber 360 may provide additional vibration damping and impact absorption capabilities to the slab. It will be appreciated that shock absorber may comprise any suitable material, such as rubber, polyurethane, or other elastomeric compounds. The shock absorber may further help distribute the load more evenly along the length of the rail, potentially reducing stress concentrations and wear on specific points of the rail or slab. The slab system 300 may be assembled in-situ (i.e. once the slab 100 has been positioned in a desired location). The shock absorbers 360 (if being used) may each be positioned on a base of a respective longitudinal groove 111, 112. Each rail 340 may then be placed within the respective longitudinal groove, before the elastomeric material 350 is poured in a space on either side of each rail (i.e. between the respective rail and the sides of the respective longitudinal groove) and left to set. Alternatively, if separate shock absorbers are not used, each rail may be suspended at a desired height within the respective longitudinal groove (e.g. using packers placed at regular intervals along the length of the groove or using a lift frame, such as a magnet-based lift frame in the case of a block rail) while the elastomeric material is poured into the groove, in order to allow the elastomeric material to flow under the rail. Figure 3 illustrates the slab system 300 installed in an urban environment as part of a rail system. In Figure 3, the slab 100 is provided on a bedding layer 370. The bedding layer may be formed from any suitable material, such as a high-flow bedding mortar (e.g. a cementbased mortar). The bedding layer 370 may be bonded to the slab, e.g. by allowing the bedding layer to set around the slab. The rails 340 may be mounted within the longitudinal grooves 120 of the slab after the slab has been positioned on (and, optionally, bonded to) the bedding layer. The bedding layer 370 may be installed, for instance, by first positioning the slab at a desired location, slightly lifting the slab 100 (e.g. using screws through threaded holes in the slab, where present) and casting the bedding layer underneath the slab (e.g. by pouring bedding material through through-holes in the slab, where present). Other approaches for installing a slab in / on a bedding layer will be apparent to the skilled person, e.g. by placing a slab upon a partially set bedding layer. The depth of the bedding layer 370 may depend on properties of an underlying structure on which the bedding layer is provided. Preferably, the bedding layer has a depth of at least 5 mm. This depth helps to ensure that the bedding layer can effectively bridge over any high points or protrusions, creating a smooth and level surface for the slab to rest upon. It also provides some flexibility in adjusting the final height and level of the slab during installation. If the underlying structure has a rough surface, the bedding layer may have a depth of at least 5 mm above an uppermost part of the underlying structure. In some examples, where each longitudinal groove 111, 112 is formed by a respective pair of longitudinal protrusions 121, 122 (as in Figure 3), portions of the slab on either side of the pair of longitudinal grooves and between the longitudinal grooves may be covered with a pavement or surface course 380. This enables other vehicular traffic to move over the rail system installation. The surface course may be formed from any suitable material, such as asphalt, mastic asphalt, macadam, tarmacadam, concrete, a resin-bound material, etc., depending on the type of traffic the surface course is intended to support. In some examples, the surface course may be formed from a material including a binder, such as stone mastic asphalt manufactured with polymer modified binder (SMA PMB), or concrete. Alternatively, a binder course may be provided between the slab and the surface course. A primer may be applied to the slab before applying the binder course or surface course. The surface course 380 may extend no higher than the longitudinal protrusions 121, 122, in order to leave the top surface of the rail (and, in particular, the groove in the rail head) exposed. Preferably, an uppermost surface of the surface course may be flush with the uppermost surface of each longitudinal protrusion, in order to provide a level road surface. Advantages of a level road surface have been previously described. Figure 4 illustrates a slab arrangement 400 for a rail system, according to an embodiment of the invention. The slab arrangement 400 comprises a plurality of slabs 100a, 100b, each slab as described above. The pairs of longitudinal grooves of adjacent slabs are aligned with one another to form a pair of longitudinal channels 410. In other words, a first longitudinal groove Illa of one slab 100a is aligned with a first longitudinal groove 111b of another slab 100b to form a first longitudinal channel, and a second longitudinal groove of the slab 100a with aligned with a second longitudinal groove of the slab 100b to form a second longitudinal channel. Figure 4 shows two slabs; however, as the skilled person will readily appreciate, the slab arrangement may comprise any number of slabs, depending on a desired length of track for the rail system. It will also be appreciated that Figure 4 only illustrates a single longitudinal groove for illustrative clarity, but in practice there will be a pair of longitudinal grooves. A respective set of rails (not shown in Figure 4) may be positioned in each longitudinal channel 410 of the slab arrangement 400. In other words, each longitudinal groove of each slab 100a, 100b may have a rail mounted therein, as described above. Each rail may extend across more than one slab; in other words, a length of each rail may be greater than a length of each slab. In this way, adjacent slabs of the slab arrangement may be coupled to one another by the rails. Preferably, each rail may be contained entirely within the respective longitudinal channel; for instance, a top surface of each rail may be flush with an uppermost surface of each slab (e.g. an uppermost surface of the longitudinal protrusions). In this way, the total height of the slab arrangement including the rails is no more than the height of the slab (e.g. no more than 15 cm). Each rail may be embedded in an elastomeric material (e.g. a polyurethane elastomeric, such as Edilon)(Sedra Corkelast®), provided in the respective longitudinal channel 410, as described above. The slab arrangement 400 may be installed as part of a rail system as described above with reference to Figure 3 (e.g. with a bedding layer provided beneath the slab and, optionally, a surface course provided on the slab on either side of each longitudinal channel). In some examples, adjacent slabs 100a, 100b may be further coupled to one another by a clamp 490. This increases a stiffness of the slab arrangement at each boundary between adjacent slabs, which may be particularly desirable where the rails provided in the longitudinal grooves are block rails (as the lower height of block rails compared to traditional grooved rails results in a lower stiffness of the slab arrangement). In particular, by mechanically connecting adjacent slabs by a clamp, a more continuous and rigid structure along the length of the track is produced, which helps distribute loads more evenly across multiple slabs, reducing localized stress and potential deformation. The mechanical coupling provided by the clamps may also help dampen vibrations that might otherwise propagate along the track structure. In Figure 4, the clamp is an elongate U-shaped clamp; however, any suitable clamp may be used for coupling adjacent slabs. The elongate U-shaped design of the clamp allows it to securely grip the edges of adjacent slabs while maintaining a low profile. This shape also provides resistance to both vertical and horizontal forces that might act to separate the slabs. The clamp may, for example, be formed from steel. This provides a sufficiently robust clamp that is light and resistant to water ingress and / or damage. Nonetheless, it is noted that other materials may be employed for the clamp, e.g., aluminum, aluminum-alloys, alloy steel and so on. In examples in which adjacent slabs are coupled to one another by a clamp 490, the distance X (Figure 2) between the outer edge of each outer longitudinal protrusion and the respective closest outer edge of the slab may, for each slab, be large enough to allow the clamp to be mounted to a portion of each slab outside the longitudinal protrusions. For instance, the distance X for each slab may be at least 5 cm. Figure 5 illustrates a slab system 1000 for a rail system, according to another embodiment of the invention. The slab system 1000 comprises a slab 500 and a pair of rails 540. Each rail is provided within a respective longitudinal groove 510. The slab 500 is similar to the slab 100 illustrated in Figures 1 to 4, but has a curved shape, in order to provide a curved section of track in a rail system. In other words, a surface of the slab on the first side of slab (i.e. the side on which the longitudinal grooves 510 are provided) is formed in the shape of an annular sector. The longitudinal grooves have a same curvature as the annular sector. This allows rails with a higher degree of curvature than can be accommodated by a straight (e.g. cuboidal) slab to be mounted in the longitudinal grooves of the slab 500. As the skilled person will readily appreciate, the radius of curvature of the annular sector may depend on a desired radius of curvature of the rails to be mounted in the longitudinal grooves. Nonetheless, it emphasized that it is not essential that the slab 500 be curved to provide curved longitudinal grooves. Rather, the slab 500 may have generally straight sides and comprise curved longitudinal grooves. This design may allow for better integration with surrounding infrastructure or adjacent straight track sections, e.g., straight-sided slabs may be easier to align and install in sequence, particularly when transitioning between curved and straight track sections. As another example, the slab may comprise have straight longitudinal grooves that are wide enough to accommodate rails with some degree of curvature. This approach may provide flexibility, allowing a single slab design to be used for rails with various curvatures within a certain range. The width of the longitudinal grooves may be selected to balance the ability to accommodate curved rails with the amount of elastomeric material needed, if such material is used to embed the rails. In Figure 5, the width of each longitudinal groove 510 is wide enough to allow a rail having a different radius of curvature than the longitudinal groove to be mounted in the longitudinal groove. The rails 530 mounted in the longitudinal grooves 510 have a higher radius of curvature than the longitudinal grooves. This increases a flexibility of the slab 500, enabling the slab 500 to be used for rails having a variety of curvatures. In this way, a small number of slab designs can be used for rails having a wide variety of curvatures. For instance, a set of five types of slab may enable rails having any curvature down to a radius to curvature of 15 m. For mounting rails having a width of 156 mm, each type of slab may have longitudinal grooves with a width W of 180 mm - 220 mm, e.g., 190 mm -210 mm, e.g., 195-200 mm (e.g. a width W of 196 mm). A first type of slab may have a length of no more than 6 m and comprise straight longitudinal grooves, enabling rails having straight rails or rails with a radius of curvature of 150 m or larger to be mounted in the longitudinal grooves. A second type of slab may have a length of no more than 4 m and comprise longitudinal grooves each having a radius of curvature of 85 m, enabling rails having a radius of curvature of 60-150 m to be mounted in the longitudinal grooves. A third type of slab may have a length of no more than 3 m and comprise longitudinal grooves each having a radius of curvature of 40 m, enabling rails with a radius of curvature of 30-60 m to be mounted in the longitudinal grooves. A fourth type of slab may have a length of no more than 3 m and comprise longitudinal grooves each having a radius of curvature of 24 m, enabling rails with a radius of curvature of 20-30 m to be mounted in the longitudinal grooves. A fifth type of slab may have a length of no more than 3 m and comprise longitudinal grooves each having a radius of curvature of 18 m, enabling rails having a radius of curvature of 15-20 m to be mounted in the longitudinal grooves. As the skilled person will readily appreciate, other sets of slab designs may be used to achieve a particular desired range of curvature. Variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. If the term "adapted to" is used in the claims or description, it is noted the term "adapted to" is intended to be equivalent to the term "configured to". Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. A slab for a rail system, the slab comprising:a pair of longitudinal grooves provided at a first side of the slab, wherein each longitudinal groove is configured to receive and support a respective rail for directly engaging with a wheel of a rail-based vehicle,wherein the slab is formed from a monolithic piece of fiber reinforced concrete and has a height of no more than 15 cm.

2. The slab of claim 1, wherein a distance between a base of each longitudinalgroove and a surface on a second side of the slab, opposite to the first side, is no more than 10 cm.

3. The slab of claim 2, wherein the distance between the base of each longitudinalgroove and the surface on the second side of the slab is between 4 cm and 6 cm.

4. The slab of any one of claims 1 to 3, wherein the slab is formed from ultra-highperformance fiber reinforced concrete.

5. The slab of any one of claims 1 to 4, wherein each longitudinal groove is formedby a respective pair of longitudinal protrusions extending from the first side of the slab.

6. The slab of claim 5, wherein a thickness of each longitudinal protrusion, in adirection perpendicular to both a direction in which the pair of longitudinal grooves extend and the height of the slab, is no more than 15 cm.

7. The slab of any one of claims 1 to 6, wherein a respective portion of the slabbordering each side of each longitudinal groove has a textured surface.

8. The slab of any one of claims 1 to 7, wherein a height of the slab is no greaterthan 12 cm.

9. A slab system for a rail system, the slab system comprising:the slab of any one of claims 1 to 8; andfor each longitudinal groove, a respective rail positioned in the longitudinal groove for directly engaging with a wheel of a rail-based vehicle.

10. The slab system of claim 9, wherein each rail is embedded in an elastomericmaterial provided in the respective longitudinal groove.

11. A slab arrangement for a rail system, the slab system comprising:a plurality of slabs, each slab being the slab of any one of claims 1 to 8, wherein the pairs of longitudinal grooves of adjacent slabs are aligned with one another to form a pair of longitudinal channels.

12. The slab arrangement of claim 11, further comprising, for each longitudinalchannel, a respective set of one or more rails positioned in the longitudinal channel.

13. The slab arrangement of claim 12, wherein, for each longitudinal channel, eachrail is contained entirely within the longitudinal channel such that a height of the slab arrangement is no more than 15 cm.

14. The slab arrangement of any one of claims 12 or 13, wherein, for eachlongitudinal channel, each rail is embedded in an elastomeric material provided in the respective longitudinal channel.

15. The slab arrangement of claim 14, wherein the elastomeric material is apolyurethane elastomeric.

16. The slab arrangement of any one of claims 11 to 15, wherein adjacent slabs arecoupled by a clamp.

17. The slab arrangement of claim 16, wherein the clamp is an elongate U-shapedclamp.

18. The slab arrangement of claim 17, wherein the clamp is made of steel.

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