What is Railway block code?

The railway block signalling bell code is a system of bell sounds used in Great Britain to communicate between manually operated Signal Boxes in implementing the railway block system. Each such signal box has a bell circuit to the boxes on either side of it along the line. The equipment consists of a plunger or tapper which when pressed, rings a single-stroke bell in a neighbouring box. That box similarly has a tapper for communicating back, so boxes have keys each of which rings the bell in a neighbouring box. The bells sound different tones, so that the signalman can tell them apart by ear.