What is "trainphone"

Trainphone was the Pennsylvania Railroad's system for voice communication between train crews on moving trains and with dispatchers, tower operators and similar. It was also used by the Kansas City Southern, and to some lesser extent on the Duluth, Missabe & Iron Range. It did not use radio, but rather electromagnetic induction. Railroads did not own any radio frequencies at the time, and the PRR's busy network found the previous methods (passing a physical paper message, or requiring a train to stop to pick up orders) insufficient. The trainphone system was first trialled in 1936 and perfected by 1943.