A heavy steel framework housed inside an interlocking tower containing a row of tall levers. Each lever is mechanically linked via rods or wires to track switches, locks, or semaphore signals outside. The frame includes complex mechanical logic bars that physically prevent a dispatcher from pulling levers in a sequence that would route two trains into each other.
Heard on the job
The old tower still uses a 40-lever frame, so the operator gets a real workout throwing the iron for the yard lead.