The modern "closed track circuit" was and remains today the single most important invention contributing to the safety of railroad travel. This circuit was invented and patented by William Robinson in the mid 1870s. In 1878, he formed the Union Electric Signal Company, This company was purchased two years later by George Westinghouse and reorganized as the Union Switch and Signal Company.

The track circuit predating the invention of the closed track circuit suffered from reliability problems inherent in its design. This early track circuit caused a relay to close when a block of track was occupied by a train - the wheels and axles of the train completing the relay circuit. Many different types of component failures would cause this circuit to malfunction. A broken rail or wire connection, a low battery or a malfunctioning relay would each cause this circuit to provide a false indication.

In the modern closed track circuit, a battery is located at one end of the block and a relay is located at the other end of the block. A train entering the track shorts both the relay and the battery causing the relay to open indicating that the block is occupied. Additionally, the failure of any component, such as the battery, connecting wire, rail or relay also causes the circuit to provide an indication that the block is occupied preventing a train from entering the block until the problem is corrected. This simple rearrangement of previously used components provides a system that is inherently failsafe.

Above. Early "open track circuit". Wheels and axles of a train complete the track circuit closing the relay.

Above. Modern "closed tack circuit". Basis of the track circuit used today. Wheels and axles of a train short the track circuit opening the relay. The resistor protects the battery by limiting the current drain from the battery when the rails are shorted.