Automatic Signalling using optical detectors for train detection is also possible. These can be used to control a block of track, a signal at one end is green and the other red, once the train passes the green signal both signals are red until the block is cleared. As the train approaches the red signal (with the block clear) the far signal will change from green to red and after a short delay the signal in front of the train will turn to green for the train to proceed. This could also be used with the servo controller for semaphore signals.