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
• 5 - 5 - 5 bells - opening signal box
• 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 section between A and B. This could be done by the guard of the train notifying the signalman or by the signalman observing the tail-lamp.)