Explain how train signals work:

Every railway line is divided up into sections called blocks where only one train at a time is allowed to be.  Signalling has kit attached to tracks to determine whether or not a train is in a section – this kit includes track circuits which send a small electrical current between the tracks and trains, and axle counters which count the wheels going in and out of a section.  Trackside signals control entry into and out of each block and also warn trains when the line ahead might be occupied and if they must reduce speed.  The earliest signals were a form of semaphore (arm up or down to tell the trains to stop or go).  Now, signals are exactly like robots with red, green and yellow lights.

Someone is the signalman who will tell the trains if they can stop or go.  Can use the soccer ball - only the train that has been signalled can have the ball and the ball keeps passing to different Cubs based on who the signalman decides.  Alternatively, can work like red light green light game and Cubs have to try and pass the signal before it changes (arm up = stop, arm down = go)