Magnetic Signal Company. Modern devices 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), and patented in 1966, the design goal of the level crossing predictor was to provide a consistent warning time for trains approaching a level crossing. Before this invention, the circuits used for activating a crossing's warning devices were very simple, activated 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 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. Technology All level 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. 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 when the computed time for the train to reach the crossing is equal to the programmed threshold time. The earliest level crossing predictors used analog computers to perform this calculation, but modern equipment uses digital microprocessors. Implementation 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 other. The tuned loops would