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First a few things about lines with ABS:
A single track line with ABS is dispatched using TimeTable and Train Order (TT&TO). In its basic form, TT&TO provides the safety against train collisions, by controlling meets and passes, simply keeping trains separated. Nowadays, Track Warrant Control or Direct Traffic Control are used instead, but serve the same purposes. When ABS is added to the equation, the ABS provides a technical and automatic system to enforce train separation. This raises the overall safety level, and allows the line speed to be raised above 49mph.
In very short terms, the ABS does the following:
Opposing Overlap ABS, in short, sets up a number of red and yellow signals to protect both ends of the train. More precisely it is 1-2 red signals and 2 yellow signals. By "pushing" this safety zone ahead of itself, two opposing trains will see yellow and red signals early enough to be brought to a safe stop even if traveling at fuull speed. Similarly the rear of the train will be protected by yellow and red signals. Strictly it is only necessary to protect the rear of the train with one red and one yellow signal, but Opposing Overlap ABS cannot determine the direction of the train. This is where APB is an improvement, allowing trains to follow each other closer.
Now for the signaling around sidings. The siding arrangement looks like this:
The switches are spring switches, their normal position being suprvised in the main track signals. The siding is not equipped with track circuits, thus the ABS does not "know" if there is a train in the siding or not. Main track signal are all permissive, i.e. their most restrictive indication is "Stop and Proceed". A train with the proper authorization may proceed past such a signal at red, at Restricted Speed. The signals in the siding are absolute signals, i.e. red means "Stop". These signals may not be passed when red, except after permission from the dispatcher or after other precautions. All signals are green as their normal aspect, i.e. when no train is on main track and switches are normal (the spring switches allow trains to move from siding as well as main track).
And now to the functions of the siding signals when a train passes the siding. As mentioned above, the first sign of an oncoming train (from the left) is signals dropping from green to yellow:
Next will be the signals at the end of the siding dropping to yellow:
As the train moves closer, so moves the red and yellow signals:
The train continues, "pushing" red and yellow signals in front. Opposing signals clear as the rear of the train pass them:
Proceeding towards the right, signals clear after the train:
Next interesting change is when signals around the leftmost siding switch clear. When the signal at the point of the siding switch goes back to green, the opposite signal on the main is released and changes to the same aspect as the siding signal, in this case green:
A few more words about the control of the signals at the end of the siding. As shown above, they behave according to the direction of the train. Technically they use the same mechanism in that they monitor the opposing signal at the point of the switch to the siding. If this signal is not green, there's a train present somewhere. The ABS detects whether this train from the one end or the other, thereby determining which way it is traveling. By always keeping one of the signals red it is not a safety problem if the determined direction of travel turns out to be wrong or the train changes direction. This may be a bit tricky to understand and maybe needs another illustration or two...
And now to the siding switches, and their influence on the signals:
In general, ABS signals supervise the switches. An ABS signal governing movement over a switch in an unsafe position, must show its most restrictive aspect. In practical terms, a check for a switch' normal position is included with a check it the track circuit, so opening a switch is that same as occupying the track that the switch is in:
Without trains to blur the effect of the open switch:
Comments, corrections and more information about ABS systems are very welcome. Email me at firstname.lastname@example.org
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Text, Images, HTML: Carsten S. Lundsten.