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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.

First developed in concept by Stanford Research 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. Unfortunately, this made for longer warning times for all trains approaching the crossing at lower speeds. Very slow (5 mph, for example) trains could receive many minutes of warning time, thus holding up highway traffic unnecessarily.

All grade crossing predictors rely on the changes in the electrical characteristics of the rails which occur as a train approaches the point at which the predictor is connected to the rails (the feedpoint). Essentially, 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.

This inductance can be measured by connecting a constant current a.c. (alternating current) source to the rails, and measuring the voltage which results. Ohm's Law tells us that with a constant current source, the voltage measured will be proportional to the impedance. The absolute magnitude of this voltage and the time-rate-of-change of that magnitude can then be used to compute the amount of time remaining before the train arrives at the crossing. (For a constant-speed train only.)

The crossing's warning devices are activated whenever the computed amount of time remaining before the train's arrival at the crossing is less than or equal to the desired (programmed) amount of warning time. However, each equipment manufacturer has their own special algorithm for determining this. The earliest grade crossing predictors used analog computers to perform this calculation, but modern equipment uses digital microprocessors to do this.

Many states in the US are now requiring the use of this type of equipment at all newly-constructed grade crossings.