The lever frame was the most common type of mechanical interlocking system. Signals and switches on the railway yard were operated remotely using levers and wires. The different colours of the levers showed what they were used for.

Semaphore levers and coupling levers for operating semaphores were red.Switch levers and derailer levers for changing switches and derailer were blue.
Lock levers for locking switches were green.
Lock levers for locking derailers were brown.