AUTOMATIC BLOCK SIGNALING
Train Operation on Single Track 302
Typical Circuit 304
In the preceding section, we have seen a combination of signal equipment which was assembled into a workable block system for multiple track, single- direction operation, using line-controlled SA-1 signals. Now, we have to get two such schemes combined on a common track for either-direction operation.
So far, we have assumed signals to be spaced braking distance apart. We have had no reason to include any track contacts in a signal control other than those located between the signal in question and the next signal in advance. If it suits our purpose, however, we can include as many additional track contacts as we wish. These additional track contacts constitute what is known as an overlap. This is the first new element encountered, and we shall expand this element as we go along.
In our illustrations, we shall show solid that part of a signal control which is affected by track circuits and dotted that portion of the signal control which has to do the advance warning. Figure 301 shows the control of signal 1 overlapped beyond signal 3 - in fact, beyond signal 4 - whereas signal 4 is overlapped beyond signal 2 but not beyond 1. If we consider only the portion of track between 1 and 4, a train in this territory is protected by these two signals, red.
Let us consider how a train would get between 1 and 4. Assume that two trains are approaching signals 1 and 4 respectively. If 1 control were not
beyond 4, both trains would proceed into this territory, each expecting to go
at least to the other end of this vital spot. Since there is an overlap, 4 will
be accepted by the westbound train, with signal 1 protecting its head end. To
avoid difficulty west of signal 1, we shall very conveniently install a switch
and permit the westbound train to stop at 2 and then proceed at restricted
speed over the switch reversed, the eastbound train meanwhile keeping clear of
Although we may not cut signal 1 control back to 4, we could, if it suited our purpose, extend 4 control beyond 1. As shown in Figure 302, to occupy the vital area between signals 5 and 8, two trains approaching these signalsiould put them red, and both trains would have to stop, call the dispatcher to get permission to proceed, proceed by flagging ahead, or proceed at restricted speed - depending on the type of signal and its place in the rule book. If we have a siding at this point, the superior train would hold the main and the inferior train would take the siding.
TRAIN OPERATION ON SINGLE TRACK
Figure 303 shows, diagrammatically, an arrangement of signals and signal control limits for an absolute permissive block signal system. This system, commonly called APB, provides full-block protection against opposing moves and allows permissive following moves. For purposes of simplifying our example, we shall assume that there is more than twice braking distance between signals 3 and 4 and between signals 9 and 14.
Figure 301. Diagram showing the overlap principle.
1 2 3 4
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1 2 3 4 5 6 7 8 9 10
Figure 303. Diagram of absolute permissive block signal system, showing signal location and controls. Horizontal scale indicates miles.
Let us suppose that we have two trains, one at Rochester and one at Syracuse. The predetermined order of their using the track between those points, established by timetable or train order, has been overlooked somewhere along the line. They start towards each other, and they arrive at signals 9 and 14 red, with braking distance for both between them. They must have left their respective stations within a matter of seconds of each other. The chances of doing this are very remote, but it could possibly happen, hence our signaling system must protect against it.
either train had entered the single track a little ahead of the other, opposing
protection would have operated and caused the opposing absolute signal 7 or 16
to go to stop. An absolute stop signal can be passed only on permission from
the dispatcher, who may be called on the telephone usually located near the
signal. In case of a failure of communications, the stop signal may be passed
by flagging ahead of a train.
So far we have used signals 7, 9, 14, and 16. Signals 10, 11, 12, and 13 are simply to expedite following moves. Signals 1, 3, 4, and 6 illustrate a
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Figure 302. Overlap signal controls at a siding.
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of signaling in a similar 4-mile stretch between Buffalo and Rochester. Signals
4 and 3 are permissive signals and may be passed at restricted speed after a
stop has been made -
permission or flagging ahead is required. The minimum signaling is sufficient
for light traffic, with following trains an hour or more apart. An occasional
train will lag, holding its following train at 4 miles for green signals. If
fleet movements are to be encountered, then we must treat our railroad as shown
in Figure 303 between Rochester and Syracuse.
Let us now examine Rochester siding for possibilities permitting a safe meet. The solid lines in Figures 302 and 303 are the same, and the meet is made under the protection of stop-and-proceed signals 5 and 8. But again we should see how the system works as we approach the siding from 3 and 10. If each red signal sent back a single yellow for approach warning, 6 and 7 would be red, 5 and 8 would be yellow, and 3 and 10 green. With such a system, trains accepting signals 3 and 10 green would find 5 and 8 red and would be in no position to do anything but overrun the signals. We, therefore, extend our approach warning system (Figure 303) and say that 3 and 10 shall be yellow, as well as 5 and 8, when 6 and 7 are red. Here we have another new element - double approach, or as it is sometimes known, green overlap.
We should mention two special conditions to be met in the application of APB. First, a feature previously considered essential was the use of a stick relay at each absolute signal to cut off the overlap and reduce the double approach to single approach for following trains, It turns out that when traffic gets heavy enough for this feature to be valuable, it is about time for cTc. Second, a problem that is present on every application is lengths of single track with insufficient braking distance between sidings to afford the protection we have shown for simultaneous entry. This is solved by the use of double or even triple approaches.
Referring to Figure 304, our circuits follow the pattern of single-direction block signaling, with the elements of protection for opposing moves, overlap, directional stick circuit, and double approach added.
for opposing moves is shown in the control for signal 1. As soon as a estbound
train passes signal 6, 6T drops, putting signal 3 to stop. When signal 3 goes
to stop, contacts in the signal open the circuit to relay 3YGP. When 3YGP opens
its front contacts, the circuit to signal 1 is opened, and signal 1 goes to
You will note that signal 6 is controlled in a similar manner, beginning with iT. Thus if, through some misunderstanding of train orders, an eastbound train left Buffalo at the precise instant that a westbound train left Rochester, signals 3 and 4 would go to stop as soon as relays 6T and iT respectively opened their front contacts. Thus, although each train would come upon a red signal, there would be sufficient braking distance to come to a stop.
Overlap is illustrated by having a front contact of track relay 6T in the control of signal 8. In a like manner 7T is included in the control of signal 5.
The directional stick circuit is one of many, but all accomplish the same purpose. As a train passes signal 4 clear, 4 track drops before slow-release signal repeater 4YGP drops. This allows time to pick up 4S relay, which is then held through its own contact, the back of the track, and the back of the YGP while the train is still on 4 track circuit. The stick remains up as long as the signal is red, and its repeater is down. Note that as we continue westward, 3 track does not pick up 3 stick, as 3 signal was red from the time the train passed 6 signal. The 3YGP being down prevented 3 stick from picking up. The use of this relay is to apply energy of such polarity to the line as to cause the signal in the rear to display yellow for a following move. If a stick relay should remain up after its YGP is picked up, thereby removing a normal overlap for opposing moves, its back contact will open the circuit of the opposing signal.
The double approach is accomplished in many ways, but the use of a green signal repeater 8GP to pole change signal 10 in the rear gives the desired results as long as only one such double approach is required at any point. When the track layout indicates that successive double approaches are required, we must resort to a line wire scheme.
The picture you should bear in mind is that of a westbound train on 4T with signals 1, 3, and 4 red; and 6 yellow. The other picture is that of trains at 3 and 10 approaching Rochester with 6 and 7 red; and 3, 5, 8, and 10 yellow.
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Figure 304. Simplified circuit showing elements of absolute permissive block system controls.