Definition ……………………………………………………………………………………………202

Signal Spacing ……………………………………………………………………………………….202

Circuits ……………………………………………………………………………………………….203

Neutral Line Control …………………………………………………………………………….203

Polarized Line Control …………………………………………………………………………..203




The Association of American Railroad’s Standard Code defines automatic block system as: “A series of consecutive blocks governed by block signals, cab signals, or both, actuated by a train, or engine, or by certain conditions affecting the use of a block.”





Figure 201 shows a piece of railroad which has been divided into blocks AB, BC, and CD. The lengths of blocks are planned according to the density of traffic that is to be handled and by the braking distance of the trains. Where traffic is heavy, blocks are relatively short, in order to permit trains to follow each other closely but safely. Where traffic is light, blocks can be longer.


For safe operation, the spacing between signals must not be less than braking distance. Braking distance means the distance it takes to stop a train which is operating at maximum permissible speed. Braking distance is a variable factor, and most railroads have conducted tests to determine what this distance is for their trains. This data is used in determining the minimum lengths of blocks, Of course, compensations must be made for grades. In actual practice, automatic block signal spacing on mainline railroads varies anywhere from 1/2 mile to 2 miles; while on rapid transit systems, spacing may be only a few hundred feet.


You might say, “Why not space these signals so far apart that there would be no question of having sufficient braking distance?” This can be and is done in territory where there are but few trains, but in places where trains have to follow each other at short intervals, it becomes necessary to make these blocks as short as possible so as to handle the traffic.


Let us consider the simplest aspects that can be displayed by a signal system protecting this piece of railroad and their meaning to an engine- man on a train. Each of these signals, 1, 2, 3, and 4, is capable of displaying a red, yellow or green light.


These aspects are defined as follows in the book of rules which govern operation of trains in automatic block territory:


Green      -       Proceed.

Yellow    -       Proceed preparing to stop at next signal.

Red         -       Stop; then proceed at restricted speed.


In order to see how these signals function for train operation, let us assume a train X has stopped in block AB, and a following train Y is approaching signal 4. Signal 4 is green; therefore, train Y will continue to operate at normal speed until it approaches signal 3, which will be yellow. Then the train will reduce speed so as to be able to stop when it has arrived at signal 2 which is red.


To keep trains moving at a high rate of speed and on closer spacing or headway, 4-aspect signaling is used. On this system, each signal has two heads mounted one above the other. The aspects and their definitions as set forth by the AAR Standard Code are:


RR - Stop; then proceed at restricted speed.

YR - Proceed preparing to stop at next signal.

YG - Proceed approaching next signal at medium speed (30 mph).

GG - Proceed.


With this system, the signals are no longer spaced braking distance apart. However, the distance from the YG aspect to the RR must be at least braking distance, and the distance from the YR aspect to the RR must be at least braking distance for 30 mph.


Let us take Figure 201 and modify it to a 3- block, 4-aspect system. Let us assume that in Figure 201 the signals are spaced just braking distance apart. With the aspects as given for this system, another signal can be located between each of the existing signals. With this modification, the signaling would then be as illustrated in Figure 202. (For practical purposes such a rough- and-ready method for conversion from 2-block to 3-block signaling would most likely be quite impractical, but it is used here to simplify our example).



Figure 201. Diagram of 2-block, 3-aspect signaling.



Figure 202. Diagram of 3-block, 4-aspect signaling.





Now let’s assume some different conditions for trains X and Y in Figures 201 and 202 and see what the advantages of the additional signals are.

With both trains as shown, should X be standing still, V would reduce speed at signal 3 in both figures.

If X were standing across joint A, Y in Figure

201 would have to reduce speed at signal 3, but V

in Figure 202 could continue at full speed up to

signal 21.

If X were standing just across joint B, Y in Figure 201 would have to reduce speed at signal 4,

but Y in Figure 202 could continue at full speed up

to signal 31

If the distance AB is, let us say, 7500 feet, the distance from red to green is 15,000 feet in Figure 201; but the distance from red/red to green/green in Figure 202 is only 11,250 feet - only 75% of the Figure 201 distance. Thus, if you can get 75 trains in 100 miles of the Figure 201 arrangement, you could get 75 trains in 75 miles of the Figure 202 arrangement - or 100 trains in 100 miles - a 331/3% increase in track capacity.


Now let’s see what circuits we are going to need to make this automatic block signaling work.

Starting off as simply as possible (see Figure 101 in Section 100, “Track Circuits, Non-Coded”) we have a very elementary arrangement where we


can get 1-block, 2-aspect signaling. Quite aside from the impracticality of having to continually creep along with our knowledge limited to just one block ahead, this arrangement has another serious disadvantage. Most blocks are of such length that it takes more than one track circuit to reach from one signal to the next. We can solve this problem in several ways. For example, as shown in Figure 203, we can use repeating cut sections. As shown in Figure 204, we can use as many track circuits as we need and take a pair of line wires through the block to check the contact positions of all the track relays. Figure 205 shows a way to increase the length of one track circuit, usually by about 50 percent.

Neutral Line Control

We can now spread our signals as far apart as we wish, but we still need additional information - “What is the position of the H relay in the next block ahead?”

Let’s find this out, as shown in Figure 206, by taking another line wire back to control a distant relay, which we shall call 0. Now, if we have H up, we know the block ahead is clear; (and if we have D up, we know the second block ahead is clear, so we can show a green signal). This circuit needs three line wires.

Polarized Line Control

The next step is to use a three-position polarized relay, thus cutting our line wire requirements to two.




Figure 203. Use of repeating cut sections in block.






Figure 204. Use of H relay to check block occupancy.

I &

Let’s pause a moment here to consider the signals we are using. Thus far we have been controlling color-light signals without moving parts - with a lamp for each colored roundel. Figure 207 shows how the GRS Type SA-1 searchlight signal, for example, changes its aspects and contact positions in response to its control circuit. Thus we can control this signal over a polarized line, and we can use its contacts to furnish a distant control for the signal in the rear.

Now let’s look at Figure 208, where we have polarized line control of SA-1 signals. This circuit does three major things.

First, it detects the condition of the track between signals 1 and 2. This is accomplished by breaking the control wires through contacts on track relays a1 and b1 and on switch circuit controller X. With any contact on a1, b1, or X open, as would be the case if a train were occupying the block between the two signals, a rail were broken, or a switch misplaced, signal 2 would display a red aspect.

Second, if signal 1 is in the stop position, relay YGP would be de-energized, causing signal 2 to display a yellow aspect.

Third, if signal 1 is yellow or green, relay YGP is energized, and signal 2 will display a green aspect.

The YGP relay is a slow-acting relay in order to prevent the control circuit for signal 2 from opening momentarily while signal 1 changes from yellow to green or vice versa. If this were permitted, signal 2 would flash from green to yellow and back to green.

To conserve electrical energy, especially if primary cells are used, it is desirable to have the signals dark, and to light them only on the approach of a train. One scheme to accomplish this is to control the light circuit through a back contact on relay AE which is in series with the control of signal 2 and will be de-energized when a train is occupying block AC. Another scheme is to reverse the relay and battery on track circuit AB and con-


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Figure 205. Center-fed track circuit.







Figure 206. Simplified circuit for 2-block. 3-aspect signaling with neutral line circuit.



trol the light circuit through a back contact on the track relay.

Switch protection and approach lighting are discussed in more detail in Section 500, “Block Signaling Adjuncts”.

Figure 209 shows a polarized line control arrangement using biased-neutral D relays and color- light signals. A biased-neutral relay picks up its

armature with one polarity only, as indicated by the arrow in the relay symbol. Current with the opposite polarity does not operate the relay. The HP relay is a slow release repeater of the H relay. It is used to prevent signal 2 from flashing red while signal 1 is changing from yellow to green. Reversal of the current through the H relay causes it to drop momentarily.
















searchlight signal contact positions (shown facing terminal block).







Figure 208. Simplified circuit for 2-block, 3-aspect signaling with polarized line circuit and Type SA-l searchlight signals.

Figure 209. Simplified circuit for 2-block, 3-aspect signaling with polarized line circuit, using biased-neutral fl relay and color-light signals.


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