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Absolute Block Manual
Differing Versions

Absolute Block Manual

Differing Versions

To date developers have included a representation of Absolute Block instruments in various sims which have been released.

Each of the versions differ in various ways. This manual covers the system used in simulations developed by Karl Latham and James Pedrick. these are:

(Simulations which contain absolute block which are not listed here use a different system, described on the respective simulations manual page)


Absolute Block

For the purposes of this manual the working of the system is demonstrated between Prestatyn and Talacre on the North Wales Coast simulation.

There are four control roundels for each block section:

Offer OF
Accept AC
Train Arrived AR
Cancel CN

Each signaller “controls” two of these roundels per block section, which are identified by their position relative to the green panel boundary line.

There are three indications for each block section:

Normal NRM Indicated white
Line Clear LNC Indicated green
Train On Line TOL Indicated red

The clearing point for each block section is shown as an overlap to the Home Signal.

The Absolute Block sections work as follows:

Here is the Prestatyn to Talacre section in its normal state. None of the control roundels are illuminated and both block indicators are indicating Normal.


When the Signaller at Prestatyn has a train to send towards Talacre, he first offers it by clicking on the “OF” roundel. A message will appear in the message box and the “AC” roundel at Talacre will now flash.


To Accept the train, the Talacre Signaller clicks the “ACP” roundel. The block indicator will change from “NRM” to “LNC”.


The Signaller at Prestatyn can now clear his section signal.


When a train enters the section, in this case 2R43, the block section will automatically change from “LNC” to “TOL”.


When the train is clear of the overlap track circuit of the home signal of the next box, there will be a message in the message box and the “AR” control roundel will flash.


The Signaller at Talacre can then click on the “AR” roundel, and the block will return to its normal state, ready for the next train.


There are two conditions where the block section can be restored to normal, from line clear, without the passage of a train - Clicking on the cancel control roundel or replacing the section signal to danger.

In both cases a timeout of sixty seconds will occur on the absolute block system. During this time, the cancel roundel will flash, and it will not be possible to do anything else with the block section until the timeout has been completed. Longer timeouts may apply to affected signals. If you click on the cancel roundel with the section signal showing a proceed aspect, it will automatically be replaced to danger.


If you wish to cancel your “offer” before the train has been accepted by the other signaller, you can do so by clicking on the "CN" roundel. If there has been no acceptance from the other signaller, then the “AC” will cease to flash and no timeout will occur.

It is possible to grant a line clear to another signaller without an offer being received. To do so, simply click the “AC” roundel with the block at normal, and the line clear state will show as detailed above.

If for any reason a track circuit between the section signal of one signal box and the clearing point of the next shows occupied, such as a track circuit failure or a shunt towards a LOS signal, the block indicator will automatically show “TOL”. When the track circuit clears you will receive a message and indication identical to the “Train out of section” state above.

Shunting into forward section

Shrewsbury has several block sections where shunting into the forward section is authorised. When the train is withdrawn from the section, in the direction of the signal box which sent it into the section, the person operating that signalbox must press the "CN" roundel to cancel the train, the block will return to normal and a sixty second timeout will apply.

Fringe Sections

Shrewsbury , Penzance and Leamington Spa & Fenny Compton both have absolute block sections which fringe to external signalboxes. These work in the normal way, shortly after you offer a train to the external signal box the block will go to Line Clear. A message will appear in the message box when you press the offer button, confirming to you that the request for a line clear has been sent. You will also receive a notification when the external signal box gives you train out of section.

Last edited by Stephen Fulcher on 28/11/2020 at 12:09