Signal; Current rail gap indicator, circa 1935

The electrified tracks on the Underground are split into sections, so that if there is a problem the power can be turned off locally, instead of having to bring the whole line to a halt. This is done by leaving gaps in the conductor rails and arranging separate power supplies to each section. When a train passes from one section to another it 'bridges' the gap. To stop this happening - which would be very dangerous if a section is supposed to be switched off - indicators are placed at gaps to warn drivers if the power has been turned off in the section ahead. Commonly known as "cherries", this signal is placed in advance of a physical gap in the current conductor rails and is illuminated in the event of the loss of traction current in the forward section. When illuminated it is to be treated as a red stop signal so as to avoid the train accidentally feeding current to the forward section.