Train Detection
- The State of the Practice
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| New predictor controlled grade crossing |
Currently, train detection for railroad operation is accomplished with an electrical track circuit. The track circuit concept dates from the early 1900’s. This technique, in general, measures the change in the electrical properties of the track circuit to extract an estimate of the train's position. For older systems, the train position is only known to be within a track segment or block. More sophisticated track circuit-based systems measure the track circuit’s impedance to determine the train’s location within the block. Modern constant warning time or predictor grade crossing control systems use this type of track circuit to calculate the train’s estimated time of arrival at the grade crossing and as a positive indication of train presence in the track block. Train speed is proportional to the rate of change of the track circuit’s impedance. Unfortunately, the only control signal available for traffic management use within the basic control system is a relay closure to convey train presence in the grade crossing approach and island block. The predictor controllers provide a relay closure at a preset time before estimated train arrival at the crossing. The relay closure only represents occupancy or impending train arrival with no indication of direction or true speed of movement. TTI has set out to explore alternative detection methodologies to supplement the traditional track circuit detection / relay closure output and means to deliver the data to a central reception point.
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| Train detection mobile unit |
A new technique for train detection is desired that can provide economical train speed, direction and presence information from a single sensor and be linked with a traffic management center. The new sensors are envisioned not to replace but augment the current vital systems in the field that provide the railroad information to the traffic signal controller. This new equipment will be installed alongside the railroad tracks but outside the railroad right-of-way. Given this stipulation, the system should have a detection range of several hundred feet. The exposed sensor equipment must be rugged, outdoors survivable and reliable. The hardware must function with no intervention after installation and setup. Train speed samples should be taken at least once per second allowing for a near continuous speed profile. The resolution of the measurements should be 1 mph and accurate over a range of 3 to 100 mph. To increase equipment mobility, a small form factor, low power design is desirable. Low power consumption enables solar power as the main energy source.
A radar-based approach appeared most attractive for delivering a semi-continuous speed profile of a train. Two different radar technologies were explored, traditional radio frequency radar and lightwave (laser). Laser systems offer the ability to report speed and distance of a target while RF-based units typically report speed only. Experiments were conducted to determine each system’s ability to acquire speed data from a passing train. Radar systems rely on a reflected signal from the target back to the receiver to operate correctly. As the train approaches, the lead car presents large perpendicular surfaces for reflection making detection easy. As the front of the train passes, the sides of the cars appear more like a skew angle planar surface. For extremely high frequency emissions (light), the vast majority of the incident energy reflects away from the receiver making detection difficult.