Signalmen at adjacent signal boxes largely communicate with each other using bell signals. Compared to voice messages, bell signals have the advantage that they are quicker to send and have less risk of being misunderstood. In the early days of railways, they also had the advantage that the equipment they use is simpler than a telephone.
The exact type of equipment used varies between signal boxes according to the signalling regulations in use. However, in general, each signal box will have a bell for each signal box that it sends trains to or receives them from, and some way to ring the equivalent bell in each far signal box. If a signal box uses token instruments, there will be a bell plunger built into each instrument. Other signalling regulations might use a 'tapper' built into a block instrument, or a separate tapper on its own.
In each case, pressing the tapper or plunger will ring a bell in the other signal box. 'Single-stroke' bells are used, so one press of the tapper causes a single 'ding' in the other signal box. The bells in each signal box will be made up of a variety of bells of different pitch and tone, so that the signalman can easily identify each bell by its sound.
It is often possible to hear at least some of a signal box's bells when standing on the platform. Remember, though, that you will nearly always only ever hear the bell signals being received. When a signalman sends a bell signal, his own bells do not ring.
Bell signals are made up of groups of beats separated by pauses. They are written down as the number of beats in each group, with dashes for each pause. For example, '3-1' means three beats, pause, one beat.
Example bell signals
The most common bell signal is probably the 'Call Attention' signal, used before nearly all other signals to alert the signalman that a bell signal is coming. It consists of just a single beat of the bell.
The largest group of signals consists of the "Is line clear..." signals, because there is a different signal for each class of train that a signalman might want to send. Some examples are:
- 3-1 Is line clear for Class B ordinary passenger train?
- 4 Is line clear for Class A express passenger train?
- 2-3 Is line clear for Class G light engine(s)?
There are also variants on these signals that indicate particular types of stock, or that are for SVRSevern Valley RailwaySVRA:Severn Valley Railway AssociationSVRS:Severn Valley Railway Society-specific operational purposes:
- 3-1-2 Is line clear for Class B ordinary passenger DMUDiesel Multiple Unit?
- 3-3-3 Is line clear for Class A/B passenger train not stopping at Bewdley?
- 2-5-1 Is line clear for Class C footplate experience train?
Bell signals are also used by signalmen to inform each other about the passage of trains and for other reasons:
- 2 Train Entering Section
- 2-1 Train Out Of Section
- 5-5-5 Opening signal box
- 7-5-5 Closing signal box
Not all codes are used by all signal boxes. E.g. Only boxes that may be switched out would use 5-5-7.