Signal 7121 and 7122 literally sit on the edge of the Great Plains. Between Shoemaker and Springer, one gets a feeling of looking over a ledge at the vast and empty land. It is a place where one can even see the curvature of the earth.

Absolute Permissive Block (APB) was introduced by General Railway Signal (GRS) of Rochester, New York and was first installed in 1911 on the Toronto, Hamilton and Buffalo (TH&B) in Ontario, Canada. In an APB system, all signals facing an opposing movement between sidings drop to red (the "absolute" part in APB) regardless of the number of blocks in front of the movement, while the signals for following movements will clear behind (the "permissive "part in APB). In effect, APB knows which direction a train is moving between sidings. This is a wonderful advantage for an automatic signal system in single track because it offers the maximum protection for opposing movements, yet at the same time expedites following movements. In other words, APB not only adds safety, but also adds capacity -- a truly win-win situation.

The underlying secret to making APB work is the stick relay. A stick relay is not a specially manufactured relay, but is rather a standard neutral relay that is energized, under certain conditions, through its own contacts (battery is feed through the front contacts and then to the coils). Every pair of signals has two stick relays, usually referred to as an east stick (ESR), for eastward movements, and a west stick (WSR), for westward movements. The two sticks are interlocked such that only one stick can be energized, or in the "stuck" position. When the system is at rest (i.e. no trains in the system), both sticks are de-energized. The way a stick relay is energized is through a sequence of de-energizing track relays at the moment a train crosses from one block to the next (the process is actually more subtle and complex). A stick that is energized will allow its signal to clear as the train advances to the next block.

To get an idea of how the system works, let's take an example of our 3 intermediate signals between Levy and Colmor. The description of the circuit system will be simplified to give an overall idea and will apply only to the westward signals. Now let us say that we have an eastward train leaving Levy. Each westward signal is held up by a local HDR relay that is tied into the signal in front of it. So for 7121, its HDR is held up (energized) by signal 7141, whose HDR in turn is held up by 7171. And finally 7171 is held up by the westward signal at the east end of Levy. The best way to visualize this system would be to imagine a series of dominos standing in a line. When our eastward train departs Levy, 7171's HDR will de-energize, which will cause 7141's HDR to de-energize, which will finally cause 7121's HDR to drop. In other words, the eastward train has pushed the first domino, causing the rest to fall backwards. In fact, the falling dominos stretch to the westward signals at the west end of Colmor. Another thing to keep in mind is that all dominos for westward signals can only fall in one direction and that would be eastward. So if you were to take the middle domino and knock it down, it would fall only to the east, while those in front of it would remain standing. As the eastward train clears each successive westward signal, the fallen domino is made to stand again (provided no train is following our eastward movement) and the signal will then clear (this is not quite true, but for now we will say that it does and will get back to why it doesn't later). So each signal will stand green as long the domino in front of it is standing.

Now let us describe a westward movement and see how the stick relay will make the circuit think it's a permissive system. Our westward train has passed 7121, causing its HDR relay to de-energize and dropping the signal to red. The train advances to signal 7141 and drops its HDR. Because 7141's domino has fallen, 7121's domino can't be picked up once the train clears signal 7141 (remember, all dominos fall to east). But at the moment the westward train passed 7141 the westward stick (WSR) energized. By doing so, it will now hold up 7121's HDR once the westward movement has passed 7141. Think of this as removing 7141's domino from the chain so that when the domino in front (7171) collapses it won't fall into 7141 (because it's not there), and so 7141 won't fall into 7121; thus, 7121 can clear. As the westward train passes 7171 (7171's WSR is energized), its domino is pulled from the chain, and once the movement clears 7171, 7141's domino is reinserted (7141's WSR is de-energized). The progression continues (take domino, reinsert behind) until the movement has cleared the west end of Levy. The stick relay (the pulled domino) at each signal prevents all the signals behind from collapsing.

Earlier in the eastward example I stated that 7121's signal does not clear, even though its HDR does, so in fact it really should clear. Though we've limited the discussion to the westward signals, the eastward signals behave in the same manner, so as the eastward movement passes signal 7122, the eastward stick (ESR) is energized. The designers of the system decided that 7121 would not be able to clear if ESR was energized. The main reason is to prevent the eastward train from making a reverse movement without doing it under restricted speed for one block; thus, protecting against any following movement. Upon further reflection, 7121 would be red regardless of the position of ESR if there was a following movement beyond Levy, so what's the point of having the ESR hold 7121 to red? Where the ESR position would be of importance would mainly be limited to signal 7171, the west approach to Levy. In this case a following eastward train moving down the main at Levy would encounter a yellow, while the leading eastward train making a reverse movement would also encounter a yellow (if not for the ESR pulling the signal to red). Two train on a potential collision course and passing their respective signals at yellow. Of course, I wasn't there when they designed the circuit, so we'll probably never know. But if you're at trackside observing the semaphores and wonder why the signal doesn't clear once the train has passed, this is why.