What is a fail safe railroad track circuit?

Answer 1

This is a track circuit built on the "fail-safe principle" whereby any break in a wire or the rail is intended, by design, to result in a safe state. The first track circuit was developed by Dr. William Robinson, and patented in the US in 1870. However, this was not fail-safe, and Dr. Robinson modified it to be a fail-safe design in 1872. A track circuit provides a continuous electrical circuit through the railroad rails to both sense a train and sense a break in the rails. This in turn controls railway signals and interfaces with other railway equipment to provide safe train operation.