My O Gauge Journal on
 Modelling the GWR

A personal Journey

Working the Block Signalling System

There are two types of block instrument arrangements in use on my railway. Terminal boxes and through boxes. The ‘terminal’ boxes consist of one pair of three position indicators, 1 three position switch, 1 bell key, and 1 bell. The ‘through’ boxes consist of two pairs of three position indicators, 2 three position switches, 2 bell keys, and 2 bells. The three positions of the indicators and the switches are labelled "Line Clear", "Normal", and "Train on Line". As the original indicators would have been almost always be galvanometers with the "Normal" position indicated by the needle having to be set in a ‘vertical’ position. But as my system uses servos and control PCBs instead, this means that the orientation of the needles doesn’t have to be set in a vertical position as you will see from my design later on. The bottom indicator of a block instrument control box controls the line used by trains leaving this Signal box for the other end of the section. The top indicator controls the line used by trains approaching the Signal box.

The indicators and switch are connected such that the switch controls the top indicator at the local Signal box and the bottom indicator on the remote Signal box. Therefore, if the switch on the local signal box control panel was turned to "Line Clear", the top indicator in the same block instrument would deflect to "Line Clear" and, at the same time, the bottom indicator at the remote signal box would do the same. The method employed is such that the indicator for the line leaving the Signal box is always controlled by the switch in the block instrument at the remote end of the section. By this means then, a Signalman would always control trains approaching his Signal box.

When the local bell key is depressed the remote bell on the other instrument panel will sound. As single stroke bells are used and the bell only sounds once per press of the button, I had to convert all the bells to behave as such for no matter how long the bell key would be held down.

The Basic Block Signalling Sequence for two signal boxes A and B
Assuming the block indicator at Signal Box 'A' is in the normal position, the Signalman at 'A' first of all attempts to obtain "Line Clear" for the train at 'A' to proceed to 'B'.

To do this, the Signalman at 'A' sends a "Call Attention" bell press, (1 beat of the bell) to 'B'. The Signalman at 'B' acknowledges the "Call Attention" by repeating the bell signal back to 'A', (this shows that the Signalman at 'B' is waiting at his block instrument). If this fails the signalman at ‘A’ waits a while and then repeats the signal or rings the box!

If the call attention command were successful, the Signalman at 'A' then asks for 'Line Clear' from 'B'. To do this, he sends the prescribed "Is Line Clear?" signal. Let’s assume that the train is a stopping passenger train, the signal would be the following bell presses, (3 pause 1). If the line is clear at 'B', the Signalman there will give "Line Clear" back to ‘A’ by repeating the bell signal (3 pause 1) while at the same time turning the switch of his block instrument to the "Line Clear" position. This will cause the indicator on his block instrument to show "Line Clear". Simultaneously, the needle on the instrument at 'A' will also move to show "Line Clear". This is a visual indication to both signalmen that "Line Clear" has been both asked for and given.

Having got "Line Clear" from 'B', the Signalman at 'A' can now clear the signals for the passenger train to depart his section. As the train leaves, the Signalman at 'A' sends "Train Entering Section" (2 bells) to 'B'. The Signalman at 'B' acknowledges this signal by repeating it and turns the switch on his block instrument from "Line Clear" to "Train on Line". This would make the needle at 'B' and the needle at 'A' point to "Train on Line".

Having received "Train Entering Section" from 'A', the Signalman at 'B' now gets "Line Clear" from 'C' for the train. This is obtained in exactly the same way 'A' obtained "Line Clear" from 'B'; first the "Call Attention" and then the "Is Line Clear?" signal are sent and acknowledged. When the Signalman at 'B' obtains "Line Clear" from 'C' he may clear the signals for the approaching train to pass through the station.

As the passenger train passes through 'B', the Signalman watches it carefully to make sure that there is nothing amiss. He also checks that the tail lamp is displayed on the rear of last vehicle on the train. If the lamp is present then the Signalman at 'B' knows that the train has arrived complete at 'B' and no vehicles have been left behind in the section.

The Signalman at 'B' then sends the "Call Attention" bell signal to 'A'. When this is acknowledged, the Signalman at 'B' can send the "Train Out Of Section" (2 pause 1) to 'A' and restore the indicator back to the vertical, "Normal" position. When the Signalman at 'A' repeats this signal, the section between 'A' and 'B' is considered clear and another train can be offered by 'A'.

This procedure is repeated as the train moves through each section.



A, B, C and D