Single line train staff, in the form of a large key

The key staff system allowed greater freedom of movement, since the key was used to unlock a box of tickets at each end of a section, which were given to engine drivers moving in the same direction along the line, with the last engine carrying the actual staff. By this means it was possible for more than one train to travel along one section of railway at a time, provided they were moving in the same direction, but it was impossible for two trains to travel in opposite directions at the same time.

With the use of the key staff there was the possibility of mistakes being made owing to the driver of a train not knowing how many trains had been allowed to enter the section in front of him, and improved methods were later introduced.