This very simple project, suitable for beginners, is a two-color (red/green) signal for a model railway. It uses a simple form of automatic operation, and if you stop the train in front of the signal it automatically switches from "green" to "red". When the train is restarted the signal automatically switches to "green" again.

To an onlooker it appears as though the signal is changing color and the train is responding to the change. In reality the train and the signal are both responding to changes in the track voltage. The signal will, in fact, go to "red" wherever the train is stopped on the layout, but this is of no practical importance, as the state of the signal is irrelevant except when the train is approaching it.