Multi-sensor synchronous data rapid acquisition vehicle for subway track area

Abstract

The invention provides a subway track area multi-sensor synchronous data rapid acquisition vehicle, which comprises a walking mechanism, a detection power system, a data acquisition system and a data processing system, and is characterized in that an equipment mounting platform is formed at the top of the walking mechanism; the data acquisition system comprises a lining structure line-scan digital camera array module, a ballast bed line-scan digital camera, a two-dimensional laser profiler, a panoramic camera and an infrared thermal imager, the lining structure line-scan digital camera array module is mounted on the bearing structure, and a vertically arranged frame structure is arranged in the middle area of the equipment mounting platform. The bearing structure is connected with the frame structure, and the ballast bed linear array camera, the two-dimensional laser profiler, the panorama camera and the infrared thermal imager are all arranged on the equipment mounting platform. The system has the advantages that the problems of relatively single data acquisition function, low efficiency, difficulty in multi-source data fusion and the like of the current subway track area can be solved.

Description

Technical Field

[0001] The invention relates to the technical field of subway tunnel data collection, and in particular to a subway track area multi-sensor synchronous data rapid collection vehicle. Background Art

[0002] There are a large number of tunnel structures, contact networks, pipelines, facilities and equipment in the subway track area, which are highly complex. Most of these facilities do not have accurate three-dimensional digital models, which brings certain difficulties to the daily operation and maintenance of rail transit and equipment asset management. In the face of the long-term maintenance needs of the huge rail transit operation network, it is necessary to rely on a comprehensive, visual, and easy-to-use spatial data base, as well as more efficient and intelligent digital management methods. In addition, with the development of the city, some rail transit tunnels are susceptible to engineering activities, environment and construction in the protection area, and structural deformation, water leakage, cracks and other diseases occur from time to time. It is particularly important for the long-term operation and safety of the subway to realize the rapid and accurate perception and positioning of tunnel structures and apparent diseases, and provide accurate and complete electronic data of management objects to the engineering monitoring and maintenance units in the tunnel section.

[0003] In the past decade, with the rapid development of 3D laser scanning, BIM, machine vision, etc., more and more scientific research institutes have begun to pay attention to reverse digital modeling of operational infrastructure. The key link of reverse digital modeling is the effective and rapid collection of data. However, the current data collection function of subway track areas is relatively single, with low efficiency and difficulty in multi-source data fusion. Summary of the invention

[0004] The purpose of the present invention is to provide a subway track area multi-sensor synchronous data rapid acquisition vehicle based on the above-mentioned deficiencies of the prior art, which can solve the current problems of relatively single subway track area data acquisition function, low efficiency, and difficulty in multi-source data fusion.

[0005] The purpose of the present invention is achieved by the following technical solutions:

[0006] A multi-sensor synchronous data rapid collection vehicle for a subway track area, characterized in that the collection vehicle comprises a running mechanism, a detection power system, a data collection system and a data processing system;

[0007] The running mechanism moves along the extension direction of the subway track under the power provided by the detection power system; the data acquisition system is electrically connected to the data processing system;

[0008] An equipment installation platform is formed on the top of the running mechanism, and the data acquisition system includes a lining structure linear array camera array module, a ballast bed linear array camera, a two-dimensional laser profiler, a panoramic camera, and an infrared thermal imager; the lining structure linear array camera array module is installed on a bearing structure, and a vertically arranged frame structure is arranged in the middle area of ​​the equipment installation platform, the bearing structure is connected to the frame structure, and the frame structure performs a lifting movement in a direction perpendicular to the equipment installation platform to drive the bearing structure to move, and the ballast bed linear array camera, the two-dimensional laser profiler, the panoramic camera, and the infrared thermal imager are all arranged on the equipment installation platform.

[0009] There are two groups of the lining structure linear array camera array modules, and each group of the lining structure linear array camera array modules has N linear array cameras. The two groups of the lining structure linear array camera array modules are arranged front and back in the longitudinal direction and are staggered at an angle in the circumferential direction. The staggered angle between the two groups of the lining structure linear array camera array modules in the circumferential direction is 360 / 2N.

[0010] There are two track bed linear array cameras, which are respectively arranged on the left and right sides of the head of the collection vehicle.

[0011] The track bed linear array camera is tilted at a preset angle toward the rear of the collection vehicle.

[0012] The two-dimensional laser profiler and the track bed linear array camera are both located in a plane perpendicular to the extension direction of the subway track.

[0013] The distances between the laser lens of the two-dimensional laser profiler and the two rails of the subway track are the same, and the end surface of the laser lens is perpendicular to the two rails.

[0014] A circular panel is arranged at the rear area of ​​the equipment installation platform, and a plurality of installation holes are opened on the circular panel. The infrared thermal imager can be fixed on different installation holes to achieve angle adjustment.

[0015] The panoramic camera is installed on the equipment installation platform through a position adjustment mechanism, and the panoramic camera is located behind the two-dimensional laser profiler. The panoramic camera can be adjusted horizontally and in rotation under the drive of the position adjustment mechanism.

[0016] Each of the track bed linear array cameras is located directly above a rail of the subway track.

[0017] The data acquisition system also includes an encoder, and the running mechanism includes four running wheels, namely two front running wheels and two rear running wheels; the encoder is installed on the running wheels, and the encoder is electrically connected to the lining structure linear array camera array module, the roadbed linear array camera, the two-dimensional laser profiler, the panoramic camera, and the infrared thermal imager to synchronously send trigger acquisition signals; the running mechanism is also provided with an anti-overturning device, which includes a laser rangefinder, an electric rod and a magnet, and the electric rod is vertically fixed to the rear of the front running wheel through a mounting frame , the magnet is installed at the bottom of the electric pole, the laser rangefinder is built into the electric pole, and a detection through hole is provided at the center of the magnet for the detection laser of the laser rangefinder to pass through so as to detect the distance to the subway track; during driving, the electric pole is in a retracted state and the distance between the bottom surface of the magnet and the upper surface of the subway track is a set value a. When the laser rangefinder detects that the distance between the magnet and the surface of the subway track is greater than a threshold value b, the electric pole is turned into an instantaneous release state so that the magnet is adsorbed on the surface of the subway track.

[0018] The advantages of the present invention are: the data collection vehicle has a relatively simple structure, a high degree of multi-sensor integration, and a high degree of data collection synchronization, which can solve the current technical problems of relatively single data collection function, low efficiency, and difficulty in multi-source data fusion in subway track areas. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is a structural schematic diagram of a multi-sensor synchronous data rapid acquisition vehicle for a subway track area in the present invention;

[0020] Figure 2 It is a structural block diagram of the multi-sensor synchronous data rapid acquisition vehicle in the subway track area of ​​the present invention;

[0021] Figure 3 It is a schematic diagram of the installation of the anti-overturning mechanism on the collection vehicle in the present invention;

[0022] Figure 4 It is an enlarged detailed view of the anti-rollover mechanism on the vehicle in the present invention.

[0023] like Figure 1-4 As shown, the marks in the figure respectively represent: running mechanism 1, detection power system 2, lining structure linear array camera array module 3, roadbed linear array camera 4, acquisition control system 5, data processing system 6, frame structure 7, two-dimensional laser profiler 8, panoramic camera 9, infrared thermal imager 10, lateral guide wheel 11, anti-overturning device 12, front driving wheel 13, rear driving wheel 14, electric rod 15, laser rangefinder 16, magnet 17. DETAILED DESCRIPTION

[0024] The features of the present invention and other related features are further described in detail below through embodiments in conjunction with the accompanying drawings to facilitate understanding by those skilled in the art:

[0025] Example: Figure 1 , 2 As shown, this embodiment relates to a multi-sensor synchronous data rapid collection vehicle for a subway track area, which includes a running mechanism 1, a detection power system 2, a data collection system, and a data processing system 6. Among them, the detection power system 2 provides power for the running mechanism 1, and the running mechanism 1 can move along the extension direction of the subway track under the power provided by the detection power system 2; the data collection system is electrically connected to the data processing system 6, and the data collection system transmits the collected data to the data processing system 6 in a timely manner, and the data processing system 6 performs unified processing. An equipment installation platform is formed on the top of the running mechanism 1. The data acquisition system includes a lining structure linear array camera array module 3, a ballast bed linear array camera 4, a two-dimensional laser profiler 8, a panoramic camera 9, and an infrared thermal imager 10. The lining structure linear array camera array module 3 is installed on the bearing structure, and a vertically arranged frame structure 7 is arranged in the middle area of ​​the equipment installation platform. The bearing structure is connected to the frame structure 7. The frame structure 7 can be lifted and lowered in a direction perpendicular to the equipment installation platform, thereby driving the bearing structure to move, and then realizing the vertical height adjustment of the lining structure linear array camera array module 3; the ballast bed linear array camera 4, the two-dimensional laser profiler 8, the panoramic camera 9 and the infrared thermal imager 10 are all arranged on the equipment installation platform.

[0026] That is to say, the data collection vehicle mainly includes four parts: a running mechanism 1, a detection power system 2, a data collection system, and a data processing system 6. Among them, the running mechanism 1 can adopt a battery-driven trolley or a rail car, which has its own power and a device installation platform; the detection power system 2 is a battery, which is used to power the data collection system and the data processing system 6; the data collection system is the core module of the data collection vehicle, integrating multiple sensors, mainly including a lining structure linear array camera array module 3, a track bed linear array camera 4, a two-dimensional laser profiler 8, a panoramic camera 9, a laser gyro combined navigation system, an infrared thermal imager 10, an encoder and a collection control system 5, adopting a modular design, and the collection control system 5 is used to control the data collection of each sensor. The data collection system is installed on the equipment installation platform on the running mechanism 1, and the data processing system 6 is used to perform data analysis on the collected data.

[0027] like Figure 1-4As shown, the running mechanism 1 is equipped with four driving wheels, namely, two front driving wheels 13 and two rear driving wheels 14, and is driven by the rear driving wheels 14. The setting is fixed by the collection vehicle control software. During driving, only the rear driving wheels 14 are allowed to brake to prevent the collection vehicle from overturning due to sudden braking. In order to prevent the risk of vehicle overturning due to sudden braking during the reverse driving of the collection vehicle, a set of anti-overturning mechanism 12 is installed at the rear end of each of the two front driving wheels 13. The anti-overturning device 12 includes a laser rangefinder 16, an electric rod 15 and a magnet 17. The electric rod 15 is vertically fixed to the rear of the front running wheel 13 through a mounting frame, and the magnet 17 is installed at the bottom of the electric rod 15. The laser rangefinder 16 is built in the electric rod 15. The center of the magnet 17 is provided with a detection through hole for the detection laser of the laser rangefinder 16 to pass through so as to detect the distance with the subway track. The detection accuracy of the single-point laser rangefinder 16 is less than or equal to 2mm, and the material of the magnet 17 can be a neodymium magnet. During the driving process, the electric rod 15 is in a retracted state and the distance between the bottom surface of the magnet 17 and the upper surface of the subway track is a set value a (1cm in this embodiment). When the laser rangefinder 16 detects that the distance between the magnet 17 and the surface of the subway track is greater than a threshold value b (1.5cm in this embodiment), the data acquisition system triggers the electric rod 15 to turn into an instant release state, so that the magnet 17 is adsorbed on the surface of the subway track, thereby preventing the data acquisition vehicle from overturning from the initial state.

[0028] Therefore, the data collection vehicle in this embodiment has a relatively simple structure, high multi-sensor integration, and high degree of data collection synchronization, which can solve the current technical problems of relatively single data collection function, low efficiency, and difficulty in multi-source data fusion in subway track areas.

[0029] In some embodiments, when the running mechanism 1 adopts a battery-driven trolley, four lateral guide wheels 11 are installed on the inner sides of the four driving wheels, and each lateral guide wheel 11 is composed of two rollers and a coil spring, wherein one end of the lateral guide wheel 11 is installed on the bottom of the battery-driven trolley and the other end is close to the inner side of the rail. The expansion and contraction amount of the coil spring is controlled within 5 mm. When the four lateral guide wheels 11 are all close to the inner side of the rail, they are used to slow down the lateral swing of the trolley during driving and improve the accuracy of data collection.

[0030] Furthermore, the method for determining the stiffness coefficient K of the coil spring of the lateral guide wheel 11 is as follows:

[0031]

[0032] In the formula, F represents one quarter of the overall weight of the collection vehicle, x represents the expansion and contraction of the coil spring, which can be 5 mm, G represents the material shear modulus of the coil spring, d represents the coil spring wire diameter, D represents the average diameter of the coil spring, and n represents the effective number of coils of the coil spring.

[0033] In some embodiments, the number of the lining structure linear array camera array modules 3 is two, and the two lining structure linear array camera array modules 3 are staggered in the extension direction of the subway track.

[0034] That is to say, the lining structure linear array camera array module 3 is composed of 2 groups of assembled linear array camera arrays, and the two groups of linear array camera modules are staggered at a certain angle to ensure that the shooting range of the linear array camera covers the entire cross-section of the subway tunnel lining structure, and the number of cameras is set according to the image acquisition accuracy. Optionally, the linear array camera can use an Eco optoelectronics wire scanning camera, which has a global shutter CMOS sensor, a maximum line frequency of up to 80kHz, a full resolution of 8192 pixels, and a minimum exposure time of 2μs. When the inner diameter of the tunnel is 5.4m and the image acquisition resolution is 0.2mm / pixel, the number of linear array cameras in each group is 6, and the two groups of linear array camera modules are staggered at an angle of 30°, which can ensure that the shooting range of the linear array camera covers the entire cross-section of the subway tunnel lining structure.

[0035] In this embodiment, two groups of linear array camera modules are installed on an H-shaped bearing structure, and the bearing structure is installed on the equipment installation platform of the running mechanism 1 through a liftable frame structure 7. The installation height of the linear array camera module can be conveniently adjusted by lifting and lowering the frame structure 7. After the height is fixed, it is fixed by a buckle. When the data collection is completed, the lining structure linear array camera array module 3 can be lowered to the lowest level to lower the center of gravity of the collection vehicle and ensure high-speed driving safety.

[0036] In some embodiments, the number of the track bed linear array cameras 4 is two, and the two track bed linear array cameras 4 are respectively arranged on the left and right sides of the head of the collection vehicle, wherein the cameras are tilted at a preset angle toward the tail of the collection vehicle.

[0037] That is to say, there are two track bed linear array cameras 4, which are located on the collection vehicle and above the track respectively. In order to prevent the track bed linear array camera 4 from blocking the laser scanning, the two track bed linear array cameras 4 are each tilted back at a certain angle θ in the longitudinal direction of the tunnel. The size of θ depends on the external dimensions of the track bed linear array camera 4 module and the two-dimensional laser scanner.

[0038] In some embodiments, the two-dimensional laser profiler 8 and the track bed linear array camera 4 are both located in a plane perpendicular to the extension direction of the subway track, that is, the track bed linear array camera 4 and the two-dimensional laser profiler 8 are installed on the same reference line of the collection vehicle head.

[0039] In some embodiments, the distance between the laser lens of the two-dimensional laser profiler 8 and the two rails of the subway track is the same, and the end face of the laser lens is perpendicular to the two rails. In other words, the laser lens of the two-dimensional laser profiler 8 is located on the vertical line of the center of the track, and the end face of the laser lens is perpendicular to the two rails without any obstruction.

[0040] Optionally, the two-dimensional laser profiler 8 can use Z+F PROFILER 9020. PROFILER 9020 is a high-speed scanner developed based on IMAGER 5010 and 2D cross-sectional laser measurement system. The point cloud scanning speed is higher than one million points / second, the cross-sectional scanning speed is up to 270 revolutions / second, and the single-point laser ranging accuracy is 0.1mm.

[0041] In some other embodiments, a circular panel is provided at the rear area of ​​the equipment installation platform, and a plurality of mounting holes are provided on the circular panel, and the infrared thermal imager 10 can be fixed on different mounting holes to achieve angle adjustment. In other words, the infrared thermal imager 10 is installed in a circular panel, and a series of mounting holes are preset in the circular panel according to the installation position angle of the infrared thermal imager 10, so as to switch the installation position and ensure full coverage of the infrared thermal imaging image of the tunnel lining structure.

[0042] In this embodiment, optionally, the infrared thermal imager 10 can use a Telops cooled infrared thermal imager, and the high-performance imaging circuit can produce a high frame rate of up to 120fps, with built-in processing algorithms such as non-uniform correction, radiation temperature, automatic control, and enhanced high dynamic range imaging, and the minimum detectable temperature difference is 18mk.

[0043] In this embodiment, the panoramic camera 9 is installed on the equipment installation platform through a position adjustment mechanism, and the panoramic camera 9 is located behind the two-dimensional laser profiler 8. The panoramic camera 9 can be adjusted horizontally and rotationally under the drive of the position adjustment mechanism.

[0044] Specifically, the panoramic camera 9 is installed behind the two-dimensional laser scanner. The panoramic camera 9 is installed on the equipment installation platform on the running mechanism 1 of the collection vehicle through a position adjustment mechanism. The position adjustment mechanism has the adjustment functions of rotation and horizontal dimensions to ensure that the circumferential plane photographed by the panoramic camera 9 is parallel to the scanning plane of the two-dimensional laser profiler 8. According to the equipment installation positions of the panoramic camera 9 and the two-dimensional laser profiler 8, and based on the equipment coordinate system of the collection vehicle, the spatial coordinate relationship between the two is constructed. According to the spatial coordinate relationship, a one-to-one correspondence between the point cloud collected by the two-dimensional laser profiler 8 and the image pixels taken by the panoramic camera 9 can be further constructed. Among them, optionally, the equipment coordinate system of the collection vehicle can be constructed with the laser scanning origin of the two-dimensional laser profiler 8 as the origin of the equipment coordinate system.

[0045] In some embodiments, the data acquisition system further includes an encoder, the running mechanism 1 includes a plurality of running wheels, the encoder is installed on the running wheels, and the encoder is electrically connected to the lining structure linear array camera array module 3, the roadbed linear array camera 4, the two-dimensional laser profiler 8, the panoramic camera 9, and the infrared thermal imager 10 to synchronously send a trigger acquisition signal. In other words, the lining structure linear array camera array, the roadbed linear array camera 4, the panoramic camera 9, the laser gyro combined navigation system, the infrared thermal imager 10, etc. are all triggered to acquire data through the digital electrical signal sent by the encoder installed on the running wheel to ensure the synchronization of data acquisition of each sensor, and each sensor is integrated and controlled through the PLC, and performs protocol communication and data storage with the industrial computer of the acquisition control system 5.

[0046] In this embodiment, the data processing system of the collection vehicle mainly includes a high-performance computer and data processing software. The data processing system receives multi-source collection data transmitted by the data collection system, analyzes the collected data based on the supporting data processing software, and uploads the analysis results to a third-party platform.

[0047] Optionally, in this embodiment, based on the conversion of the device coordinate system, the engineering coordinate system and the geodetic coordinate system, a spatial pixel-level mapping relationship between the linear array camera image, the two-dimensional laser scanner point cloud, the panoramic camera image and the infrared thermal imaging image can be constructed, and associated with the positioning data of the positioning system constructed by the laser gyro combined navigation system and the encoder to realize the fusion of multi-source collected data.

[0048] In this embodiment, based on the apparent grayscale image and infrared thermal imaging image of the tunnel lining structure collected by the collection vehicle, machine learning, image recognition and other methods can be used to identify and detect structural defects such as cracks, water leakage, spalling, and steel corrosion in the tunnel lining structure through the data processing system of the collection vehicle.

[0049] In addition, based on the laser point cloud data and panoramic camera data collected by the collection vehicle, through AI automatic recognition or manual marking, the labeling of communication, power supply, engineering, and vehicle professional facilities and equipment and the measurement of tunnel contour deformation can be achieved.

[0050] In addition, based on the multi-source fusion data collected by the collection vehicle, through algorithms such as spatial interpolation and image point cloud registration, the reverse reconstruction of the spatial data of the track area structure and facilities and equipment can also be achieved.

[0051] In summary, the beneficial effects of the present invention are: the data collection vehicle has a relatively simple structure, a high degree of multi-sensor integration, and a high degree of data collection synchronization, which can solve the current technical problems of relatively single data collection function, low efficiency, and difficulty in multi-source data fusion in subway track areas.

[0052] Unless otherwise defined, the technical or scientific terms used herein shall have the usual meanings understood by persons of ordinary skill in the field to which the invention belongs. The words "first", "second" and similar terms used in the patent application specification and claims of the present invention do not indicate any order, quantity or importance, but are only used to distinguish different components. Similarly, words such as "one" or "one" do not indicate a quantity limitation, but indicate the existence of at least one. Words such as "connect" or "connected" are not limited to physical or mechanical connections, but may include electrical connections, whether direct or indirect. "Up", "down", "left", "right" and the like are only used to indicate relative positional relationships. When the absolute position of the object being described changes, the relative positional relationship also changes accordingly.

[0053] The above is a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A multi-sensor synchronous data rapid collection vehicle for subway track areas, characterized in that: The data collection vehicle includes a running mechanism, a detection power system, a data collection system and a data processing system; The running mechanism moves along the extension direction of the subway track under the power provided by the detection power system; the data acquisition system is electrically connected to the data processing system; An equipment installation platform is formed on the top of the running mechanism, and the data acquisition system includes a lining structure linear array camera array module, a ballast bed linear array camera, a two-dimensional laser profiler, a panoramic camera, and an infrared thermal imager; the lining structure linear array camera array module is installed on a bearing structure, and a vertically arranged frame structure is arranged in the middle area of ​​the equipment installation platform, the bearing structure is connected to the frame structure, and the frame structure performs a lifting movement in a direction perpendicular to the equipment installation platform to drive the bearing structure to move, and the ballast bed linear array camera, the two-dimensional laser profiler, the panoramic camera, and the infrared thermal imager are all arranged on the equipment installation platform.

2. The collection vehicle according to claim 1, characterized in that: There are two groups of the lining structure linear array camera array modules, and each group of the lining structure linear array camera array modules has N linear array cameras. The two groups of the lining structure linear array camera array modules are arranged front and back in the longitudinal direction and are staggered at an angle in the circumferential direction. The staggered angle between the two groups of the lining structure linear array camera array modules in the circumferential direction is 360 / 2N.

3. The collection vehicle according to claim 1, characterized in that: There are two track bed linear array cameras, which are respectively arranged on the left and right sides of the head of the collection vehicle.

4. The collection vehicle according to claim 3, characterized in that: The track bed linear array camera is tilted at a preset angle toward the rear of the collection vehicle.

5. The collection vehicle according to claim 3, characterized in that: The two-dimensional laser profiler and the track bed linear array camera are both located in a plane perpendicular to the extension direction of the subway track.

6. The collection vehicle according to claim 1, characterized in that: The distances between the laser lens of the two-dimensional laser profiler and the two rails of the subway track are the same, and the end surface of the laser lens is perpendicular to the two rails.

7. The collection vehicle according to claim 1, characterized in that: A circular panel is arranged at the rear area of ​​the equipment installation platform, and a plurality of installation holes are opened on the circular panel. The infrared thermal imager can be fixed on different installation holes to achieve angle adjustment.

8. The collection vehicle according to claim 1, characterized in that: The panoramic camera is installed on the equipment installation platform through a position adjustment mechanism, and the panoramic camera is located behind the two-dimensional laser profiler. The panoramic camera can be adjusted horizontally and in rotation under the drive of the position adjustment mechanism.

9. The collection vehicle according to claim 3, characterized in that: Each of the track bed linear array cameras is located directly above a rail of the subway track.

10. The collection vehicle according to claim 1, characterized in that: The data acquisition system also includes an encoder, and the running mechanism includes four running wheels, namely two front running wheels and two rear running wheels; the encoder is installed on the running wheels, and the encoder is electrically connected to the lining structure linear array camera array module, the roadbed linear array camera, the two-dimensional laser profiler, the panoramic camera, and the infrared thermal imager to synchronously send trigger acquisition signals; the running mechanism is also provided with an anti-overturning device, which includes a laser rangefinder, an electric rod and a magnet, and the electric rod is vertically fixed to the rear of the front running wheel through a mounting frame , the magnet is installed at the bottom of the electric pole, the laser rangefinder is built into the electric pole, and a detection through hole is provided at the center of the magnet for the detection laser of the laser rangefinder to pass through so as to detect the distance to the subway track; during driving, the electric pole is in a retracted state and the distance between the bottom surface of the magnet and the upper surface of the subway track is a set value a. When the laser rangefinder detects that the distance between the magnet and the surface of the subway track is greater than a threshold value b, the electric pole is turned into an instantaneous release state so that the magnet is adsorbed on the surface of the subway track.

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