Why are there Approach Signals?

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As you may have read, approach signals are visually indistinguishable from automatic signals, but, like home signals, are controlled by an interlocking tower. You may wonder why they exist at all. The following diagram and explanation should motivate the need for approach signals. In this diagram, the lines represent tracks, and the little circles with numbers represent signals. Trains go from the left to the right. These signals are roughly 200' apart. Only relevant signals are shown. The colors represent the aspects of each head of each signal. The switch is set for Track 2.

Even though there are no trains on the diagram, it should be clear why the home signal labelled "4" ("signal 4", or "4 signal", as signal design and operating personnel often say) is at "stop and stay", "red over red". As the switch is set for Track 2, trains may not enter from track 1, lest they be derailed, damage the switch, or both, or even possibly collide with a train coming in from track 2. If the switch is set for track 2, passing signal 4 is absolutely prohibited, in any way at all in any circumstance, so it must be (absolute) "red over red" (please see home signals for home signal aspects and indications.)

However, merely signal 4 being at "stop" is not sufficiently safe. Were the switch as shown, and a train to come in on track 1 at full speed and hit signal 4, it would indeed be tripped by the train stop of signal 4, and eventually come to a halt. But trains requires many hundreds of feet to stop, and by the time the train stopped, an accident would have already happened when the slowing train encountered the switch.

Therefore, when a switch is set the "wrong way" for a given track, enough signals in advance of the switch on that track must be red so that if a train at full speed were tripped by the train stop of the first of them, it would grind to a halt before getting near the switch. For signal 4 to succeed in its role of protecting the switch when it is set for the other track, it must be sure that trains are slow or stopped when approaching it. That is why signals A2 and B2 need to be controlled by the interlocking tower -- they must be set and guaranteed to be red before the switch can be reversed (set to track 2). They are always controlled together, by the same lever (lever 2, see interlocking), and hence are designated A2 and B2. They make sure that trains cannot approach signal 4 at speed when the switch is set reverse.

You might be asking yourself, "Why can't you move signal 4 far back enough such that this is not the case?" Well, the placement of signals is determined by how close the designers want trains to approach each other, and in this case, how close trains can get to the switch, so it has to be that close, and more signals are required. There is nothing wrong per se with a train coming all the way up to signal 4 when the switch is set against it, just not at speed; in fact it is desirable -- trains should be allowed to go as far as they safely can. That is why (except on that ol' "old IRT") A2 and B2 are not themselves home signals -- even when they are red, it is safe to fully stop and proceed past them slowly with automatic key-by. Having stopped at an approach signal, a train cannot build up enough speed to endanger the switch if tripped by the home signal. This constraint, too, is paramount when the placement of approach (and home) signals is designed. As you can see, the safe placement of signals requires balancing of many factors and very substantial expertise.

Note that it is not adequate to simply make the approach signals red when the switch is thrown -- like home signals, they must be set and ensured to be red before the switch can be moved. And if there is already a train within or approaching them at the time the tower operator decides to reverse the switch, even that can be seen to be insufficiently safe ("the proof (and the solution) is left to the reader":). Please see "interlocking" for a further discussion of these issues, and investigate the note below if you find that this whets your appetite for even more laborious technical detail.

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Those who find themselves drawn to and fascinated by NYC Subway Signalling would do well to check out the NY City Signal and Interlocking Simulator (NXSYS) by the author of this page, which not only includes far more detailed information on subway signalling, but a complete implementation of it down to the level of electrical relays, as well as real-time track graphics and more.