A grade crossing predictor is an electronic device which is connected to the rails of a railroad track, and activates the crossing's warning devices (lights, bells, gates, etc.) at a consistent interval prior to the arrival of a train at a grade crossing[1] .
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First developed in concept by the Stanford Research Institute in the late 1950s at the request of the Southern Pacific Company (the Southern Pacific Railroad, now merged into the Union Pacific Railroad), the design goal of the grade crossing predictor was to provide a consistent warning time for trains approaching a grade crossing.
Previously, the circuits used for activating a crossing's warning devices were very simple, and activated them whenever a train came within a fixed distance (hundreds or thousands of feet) of the crossing. This method required that the crossing be designed to accommodate a train approaching at the track speed limit, which leads to a longer warning times for trains approaching the crossing at lower speeds. Very slow trains could have many minutes of warning time, thus delaying highway traffic unnecessarily.
All grade crossing predictors rely on the changes in the electrical characteristics of the rails that occur as a train approaches the point at which the predictor is connected to the rails (the feedpoint). A railroad track occupied by a train or other electrical shunt can be viewed as a single-turn inductor shaped like a hairpin. As the train approaches the feedpoint, the area enclosed by the inductor diminishes, thus reducing the inductance[2] .
This inductance can be measured by connecting a constant-current alternating current source to the rails, and measuring the voltage which results. By Ohm's Law, the voltage measured will be proportional to the impedance. The absolute magnitude of this voltage and its rate of change can then be used to compute the amount of time remaining before the train arrives at the crossing, assuming it is running at a constant speed.
The crossing's warning devices are activated as soon as the computed time until the train reaches the crossing reaches some programmed threshold. The earliest grade crossing predictors used analog computers to perform this calculation, but modern equipment uses digital microprocessors.
A predictor includes a short island track which just covers the width of the level crossing.
A predictor circuit in the middle of nowhere is usually terminated with a dead short across the rails at the outer ends. This assumes that there are no ordinary track circuits for block signalling purposes.
Two predictor circuits may overlap, with tuned circuits used for one predictor to jump over the over. The tuned loops would be a dead short for one predictor, and a open circuit for the other.
Many states in the US are now requiring the use of this type of equipment at all newly-constructed grade crossings.