High Quality Content by WIKIPEDIA articles! The railway block code is a system of bell rings used in Great Britain to communicate between manually operated Signal Boxes (the system is not used in modern power signal boxes). Each signal box would have a bell circuit to the box on either side of it along the line. The equipment would consist of a plunger or key (rather like a Morse key) which if pressed, rings the bell in the neighbouring box. The neighbouring box would have a key for communicating back, therefore each box has a key which rings the bell in the other box. Where a signal box communicates with more than one other box (in either direction along the line, and possibly along a branch line too), each bell would have a different tone, so that the signalman could tell them apart by ear.