Interlock — A Signal Box, 1910

You are the signalman. An illuminated enamel diagram shows the road above; below stands a brass frame of fourteen painted levers, tied together underneath by iron — and that iron is the puzzle. A signal will not come off until the points beneath it lie right and their lock is bolted. Pull a lever the interlocking forbids and it thunks dead in your hand and names the lever in the way. Beyond the box there is only the block bell: 4 for an express, 3-1 for a passenger, 2 when a train enters the section. Twelve turns of duty, graded on punctuality, safety and protocol.