Railway signal
In Britain, modern signalling uses a system similar to road traffic lights. The signals are coloured lights mounted on a pole. A red signal means 'do not go further'. A yellow signal gives a warning to the driver that the next signal is showing a red.  Some signals can show two yellow lights, known as a "double yellow" and are warnings to the driver that the next signal is showing a yellow.  A green signal means 'you can continue'.