>>1176846cont'd
Trains that ran on the bridge railway had a cab signalling system installed, which displayed speed limits of 11, 17, 25 and 35 mph. If a train got closer to the train ahead, the cab signalling would indicate to the driver that he'd need to reduce the speed. This system worked by using very short block sections of 250 to 500 ft (more or less 100-200 m), and a transponder system that sent an electric signal through the rails. Behind each train, there would be a "tail" of progressively higher speed limits in each block sections: 11 mph in the block section occupied by the train, 17 the next one, then 25 and then 35. On the bridge railway no regular signals were used, except at switches. This was conceptually a rudimentary forerunner of present-day distance-based signalling.
In this Key System promotional film, from which pic related is taken, you get to see the cab signalling in action:
https://www.youtube.com/watch?v=kclGcfRMQ_I 