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First, A Few Terms: Signal Aspects, Names And Indications
Signals are the "traffic lights" of the railroad. They help regulate traffic by slowing down trains when they need to be slowed down, and stopping them when they need to stop.
On the streets, we have to deal with 5 different basic traffic lights: green, red, yellow, flashing green and flashing red. Each shows us a different "view," such as the view of a single flashing red light bulb, and each has a different "name," such as "red" or "flashing red." Similarly, on the railroad, we have different "views" or ways that a signal looks, and we have different names for them. The views are called signal aspects, while the names are simply signal names. So, on the street, a single lit red bulb would be the aspect of a red light, while "red light" would be its signal name.
Each signal tells us to do something. On the street, the red light tells us to stop. The yellow light tells us to start slowing down as the light is about to turn red. These actions that the signals require us to take are called signal indications: The signals indicate something to us.
It is common for beginners to confuse these terms, signal aspects, names and indications. If you think of them the way we explained above, you should have no problem.
As we said, on the streets, we have to deal with 5 different aspects: green, red, yellow, flashing green and flashing red. On the railroad, we deal with some 118 such different aspects! Before you panic, however, and start looking through the newspaper job classifieds, I have to say that the aspects are so many, because there are many ways to show any given signal name. For example, as you can imagine, the aspect of a single red light bulb is a "stop signal." For reasons that will become clear later, on the railroad, we can show a "stop signal" with one red bulb, two red bulbs, or three red bulbs. That's three different aspects for the same signal name - Stop Signal. If you add the different ways we have to show each signal name -- the different aspects -- you get about 118. It's that simple.
In this section, we will talk about the different signals and their names, and we will discuss in plain language their meanings. Bear in mind that the meanings below are NOT the NORAC indications, which are more complicated. For now, all we want to understand is the basic meanings, in plain language, and so in this section we use the official names of the signals, but not the official indications.
There are basically three types of signals:
1. Signals That Bring You To A Stop.
These signals warn you of conditions up ahead that require you come to a stop or to be prepared to stop. That may be because there are other trains up ahead, or the dispatcher is not yet ready for you, or other reasons. Their names are:
Stop Signal. This signal tells you to stop -- plain and simple. You go through this one, and you are looking at a 30-day unpaid leave.
Stop And Proceed. This signal tells you to come to a complete stop first, and then proceed, but at Restricted Speed. I guess, the equivalent on the streets is the flashing red that requires you to stop your car, watch both ways, then proceed.
Restricting. This signal tells you to proceed, but at Restricted Speed. This means that you can proceed, alright, but slowly and being ready to stop at any upnormality -- as explained by Rule 80, the almighty Restricted Speed rule.
Approach. This signal tells you that a Stop Signal (or Stop and Proceed) is up ahead: You are "Approaching" a Stop Signal. For obvious reasons, the signal also requires you to start slowing down.
Advance Approach. This signal tells you that a Stop Signal (or Stop and Proceed) is two signals down the road. It gives you an Advance warning of an Approaching Stop Signal. The signal also requires you to start slowing down.
Medium Approach. This signal is like an Approach, and tells you that a Stop Signal (or Stop and Proceed) is up ahead. It also tells you that you need to regulate your speed to Medium Speed (30mph) right away. We will see later why.
Slow Approach. This signal is like an Approach, and tells you that a Stop Signal (or Stop and Proceed) is up ahead. It also tells you that you need to regulate your speed to Slow Speed (15mph) right away. We will also see later why.
2. Signals That Regulate Your Speed
Often, trains must slow down because of conditions up ahead that require them to move slower -- notably diverting moves over switches whose geometry requires the train to slow down -- or else derail! First come the Clears:
Clear. This tells you to proceed: All's Clear ahead. You can go as fast as the track speed at that location allows you.
Limited Clear. This tells you to proceed, but at Limited Speed (45mph): All's Clear ahead, but don't go faster than Limited Speed.
Medium Clear. This tells you to proceed, but at Medium Speed (30mph): All's Clear ahead, but don't go faster than Medium Speed.
Slow Clear. This tells you to proceed, but at Slow Speed (15mph): All's Clear ahead, but don't go faster than Slow Speed.
Cab Speed. This one tells us to proceed, but does not give a speed directly. It tells us to proceed at the speed indicated by the Cab Signal apparatus onboard the engine. So, proceed at the Cab-Signal-Apparatus-indicated Speed. (Sorry for this mouthful.)
So far, so good. But, what if you are doing 100mph and are coming up to a switch that requires you to do 15mph? You come up to a Slow Clear signal, pour on the brakes like mad and burn the brake shoes, but there is no way to slow down by that much, that fast. So, we have signals that bring your speed down:
Approach Limited. This tells you to proceed, but start slowing down to Limited Speed because you need to be doing Limited Speed by the next signal. You are Approaching a signal that requires Limited Speed.
Approach Medium. This tells you to proceed, but start slowing down to Medium Speed because you need to be doing Medium Speed by the next signal. You are Approaching a signal that requires Medium Speed.
Approach Slow. This tells you to proceed, but start slowing down to Slow Speed because you need to be doing Slow Speed by the next signal. You are Approaching a signal that requires Slow Speed.
Medium Approach Medium. This oddball signal tells you to proceed at Medium Speed and Approach the next signal at Medium Speed also. We use this in cases where we have to go over a set of Medium Speed switches, and then, later on, we also come to a place that requires Medium Speed; the signal tells us not to hurry and speed up.
Now, the two combination signals we have seen that lead you to a stop and slow you down, Medium Approach and Slow Approach, also belong to this list. Since they warn you of an impending stop, I lumped them with the signals that lead you to a stop. Now that you learned the "slow-downers," go back and look at them again to see how they also slow you down. You could say that in basic terms the following math equations are true:
Medium Approach ≅ Medium Clear + Approach.
Slow Approach ≅ Slow Clear + Approach.
And while at it, this is also true:
Medium Approach Medium ≅ Medium Clear + Approach Medium.
Notice, however, how I used the ≅ math symbol which means "approximately equal." This is because the corresponding indications don't necessarily match perfectly. Nevertheless, that's a quick way to remember their meanings.
3. Oddball Signals
On our territory, you may also see this one:
Clear To Next Interlocking. This is used when your train has a cab signal failure which means that the onboard Cab Signal Apparatus is inoperative. In a certain type of territory, called rule 562 territory, where there are no wayside automatic signals and cab signals are all you have, the dispatcher has to have a way to tell you there is no train up ahead of you and the track is Clear until the Next Interlocking.
That's it! Now you understand your signals! Pretty simple, isn't it?
Now you can study the actual indications which should be much easier to digest.
Once you are familiar with the meaning of signals, read my Signal Progressions guide which will help you understand how signals are used with each other.