>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.

And at Suisun-Fairfield, Redding, and a whole host of other places. When the UP took over, they said it couldn't be done, regardless of the fact that the SP had been doing it for over 30 years.