Railway block code

The railway block code is a system of bell rings used to communicate between manually operated signalboxes. (The system is not used in modern power signalboxes.) Each signalbox 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 signalbox communicates with more than one other box (in either direction along the line, and possibly along a junction line too), each bell would have a different tone, so that the signalman could tell them apart by ear.

Although codes varied from region to region the following shows a few selected UK Western region bell codes:

• 1 bell - call attention
• 2 bells - train entering section
• 2 - 3 bells - (2 bells, pause, 3 bells) is line clear for light engine?
• 3 - 1 bells - is line clear for stopping passenger train?
• 4 bells - is line clear for express passenger train?
• 2 - 1 bells - train arrived
• 3 - 3 bells - blocking back
• 5 - 2 bells - release token
• 2 - 5 bells - token replaced
• 6 bells - obstruction danger
• 16 bells - testing bells and instruments

These would be used as follows (assuming a simple exchange between two signalboxes on a plain section of line) between two signalboxes A and B. Assume a train is to pass from the block controlled by A to the block controlled by B. This must be agreed and permitted by the signalman in box B. Here is the sequence of bell codes and associated actions:

1. Box A sends 1 bell to box B. (Call attention.)
2. Box B replies with 1 bell to box A. (Acknowledgement, indicating readiness.)
3. Box A sends 4 bells to box B. (Is line clear for express passenger train?)
4. Box B sends 4 bells to box A. (Acknowledgement, giving permission for box A to signal the train into the block controlled by B.)
5. (Slightly later.) Box A sends 2 bells to box B. (Train entering section.) This is sent as the train passes box A and enters the block controlled by B.
6. Box B sends 2 - 1 bells to box A. (Only after confirming that the train has arrived complete and is no longer in the block controlled by A. This could be done by the guard of the train notifying the signalman or by the signalman observing the tail-lamp.)