>
> Years ago, with the railway system on the Bay
> Bridge, it got to be 'really' interesting because
> when trains are operated with DC Voltage, you must
> use AC track circuits, and then the fun begins...
>
> KRK

AC track detection circuits were very commonly used where propulsion power was DC. All that was needed was a coil connecting the two rails to keep the DC power out of the track signal system and to allow both rails to be used for power return circuits.

One of the more unusual signal control systems was on SP San Francisco commute train tracks. A grade crossing was at the end of the station platform. Crossing gates would go up when the train was stopped at the platform. Then when the engineer got a go signal from the conductor, he would blow the locomotive horn. A microphone picked up the sound of the horn and caused the gates to close so the train could proceed.