Rails/Railroad/Railroad preemption technology
P Minch wrote at 2008-07-09 16:45:53
There are three basic types of track circuits used at highway crossings. In the simplest, occupancy of the track triggers the signals with additional circuitry to shut them off when the train is moving away on the other side. The length of the track circuit is dependent on the maximum train speed and the desired warning time.
There are also motion detectors that will activate the signals only for a moving train.
Then there are the predictors that not only detect motion, but calculate how long the train will take to arrive at the crossing and keep the warning time constant regardless of train speed.
I hope this answers your question.
Jim wrote at 2013-12-31 03:49:21
First of all, many non-electrified roads use grade crossing "predictors" which do exactly how the OP suggests: they measure the speed of the approaching train and adjust the warning time accordingly to provide a "constant warning time" regarless of approach speed. Special types of rail shunts are placed at the approach limits of the crossing in either direction and create a termination point. When the train passes these shunts, the predictor begins calculating the train speed and decides when the activate the crossing.
There are typically signs or markers along the ROW to indicate to the engineer where the approaches begin. Operating rules tell the engineer that s/he cannot speed up once on the approach. Also, if the train stops before reaching the crossing, the predictor will "time-out" and allow the crossing to reset/become inactive.
These devices do not work well in electrified railroads where the rails are used to return the propulsion current from the traction motors to the substations.