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.