Prior to the widespread availability of electricity, switches at heavily traveled junctions were operated from a signal box constructed near the tracks through an elaborate system of rods and levers. The levers were also used to control railway signals to control the movement of trains over the points. Eventually, mechanical systems known as interlockings were introduced to make sure that a signal could only be set to allow a train to proceed over points when it was safe to do so. Purely mechanical interlockings were eventually developed into integrated systems with electric control. On some low-traffic branch lines, in self-contained marshalling yards, or on heritage railways, switches may still have the earlier type of interlocking.
Prior to the widespread availability of electricity switches at heavily traveled junctions were operated from a signal box constructed near the tracks through an elaborate system of rods and levers The levers were also used to control railway signals to control the movement of trains over the points Eventually mechanical systems known as interlockings were introduced to make sure that a signal could only be set to allow a train to proceed over points when it was safe to do so Purely mechanical interlockings were eventually developed into integrated systems with electric control On some low-traffic branch lines in self-contained marshalling yards or on heritage railways switches may still have the earlier type of interlocking