Signals

In terms of running railways, signals are mechanical or electrical devices erected beside a railway to pass information relating to the state of the line ahead to the engine.

Signalmen use these to alert the engine's crew to stop when another engine passes so an accident won't be caused. However, sometimes engines don't pay attention.

One time, Henry was passing with the Flying Kipper and he passed a yellow signal and they were gonna stop, but the next signal was down. But then he collided with a goods train thanks to the snow and ice.

Because of this, Sir Topham Hatt ordered all of the signals to go upward instead of down. Gordon and James played a trick on Percy about "backing signals".

However, in recent episodes, the signal for "go" has changed back to downwards.