Sign; Sign indicating the negative current rail section switch number 99A, circa 1970

Sign for a negative current rail section idicating the number of the switch and the track that it would elecrtically isolate, (99A northbound local). This sign would have been matched by a positive current rail section sign, (also in the collection).

The sign would have been on the exterior door of a knife switch case on the Underground. The origin of the sign is not known, but this type of plate is still in widespread use throughout the London Underground system.