Signal Series – ABS, Part 1

ABS Signals – An Introduction

There are a few posts I’ve been putting together, but I realized that an overview of the signal system on MAX was really needed in order for the posts to make sense.  Putting all of the signal descriptions in one post would be information overload, so here it is in (hopefully) easy-to-digest segments.

I’m starting with  the Automatic Block Signal System, or ABS for short.  A lot of the MAX alignment uses this type of signaling.

Blocks, Aspects, and Indications

Before describing the details of the signals, let’s cover some of the terminology. If you didn’t click on the above link explaining what ABS does, the briefest explanation is that it’s a system of rail signals that gives information about the alignment ahead of you, including how the switches are set and if there is a train in front of you.

An important concept to understand is a “block”. It’s pretty straightforward – a block is the distance between two ABS signals. The exact distance that a block cover in terms of miles/feet/etc can vary, but no matter how long or short it is, the distance between two ABS signals is always one block. Once you pass a signal, you’re in a new block until you pass the next ABS signal.

Onto the signals themselves – the most important words associated with these are aspects and indications. An aspect is the part of the signal that is lit. In other words, the part that’s considered a “light” to car traffic. So while in your car you have a green light to go, a train will have a green aspect.

And an indication is what that aspect means. For example, when a train has a red aspect on a signal, its indication (what it means) is STOP.

So here’s a basic, one-headed ABS signal that can display 3 aspects:

Stop, signal 22Red aspect on signal 22, Lloyd Center eastbound

A red aspect on this signal (as shown) indicates “stop.” That’s the easy one!  This picture was taken right after a train left Lloyd Center heading eastbound, and it entered the block that is between Signal 22 and the next signal. If a train came into Lloyd Center right now and saw this red aspect, they would know that meant that the train in front of them is still in that block. Therefore, that following train is not clear to proceed past it until that first train has moved into the next block.  At that point, the signal will become…

signal 72Yellow aspect on signal 72, Gateway westbound

A yellow aspect on this signal indicates “clear for one ABS block on the primary route” – meaning from this signal to the next one, there are no trains there. However, in the block after that, there is a train there (so the signal after this one will be red until that train leaves).  When a rail operator passes a yellow ABS signal in a high speed area, such as coming out of the tunnel going westbound, they won’t bring the train up to the full speed limit because the next signal will be red, so they will slow down giving the train in front of them time to clear its block.

W754Green aspect on signal W754, Beaverton Transit Center eastbound

A green aspect on these signals indicates “clear for two ABS blocks on the primary route” – meaning not only are there no trains between this signal and the next one, there are also no trains between *that* signal and the one after it.

Here’s another way of looking at it:

Green ABS diagramYou are the operator of the train on the left side of the picture.  The direction of travel is from left to right, and there is a train in front of you.  There are two ABS blocks between you and that train, so the signal you will see will be green.  You proceed through it and enter that first open block.

Yellow ABS diagramIf that train in front of you hasn’t moved, when you get to your next signal, it’s going to be yellow.  This lets you know that you have one open ABS block clear between you and the other train.  You proceed through it and enter the new block.

Red ABS diagramIf that other train still hasn’t moved, when you come up to the next signal, it will be red.  This tells you that the block in front of you is still occupied by your leader, and you are not clear to proceed past this signal into that block.  Two trains cannot occupy the same block at the same time, so you must stop until you get a clear to proceed.

Some ABS signals, like this one, can only display 2 aspects. That means there is a platform in the next ABS block.

Two aspect signalYellow aspect on signal W1534, Fair Complex eastbound

So there are no trains between you and the next platform (which is Hawthorn Farm) – there may or may not be a train in the block after that platform, but there will be another signal there that will tell you that information when you get there.

There is also one more aspect that some ABS signals can display – the lunar aspect:

Signal 14, lunarLunar aspect on signal 14, Steel Bridge westbound

This one can be a little confusing – it indicates “Proceed with caution, tracks may not be clear.” This one will tell you how your switches are set and which route you are going on, but it does *not* tell you if there is a train ahead of you. Although this is an ABS signal, it is used in pre-empt territories and sometimes at the border between ABS & pre-empt territories- you’ll see lunars at both the Hatfield terminus and the Jackson turnaround as well as on the Steel Bridge, which are all in pre-empt territories. Lunars also are used to move Blue Line trains from Gateway to Burnside and from Burnside to Gateway.

Up next – more with ABS signals!

8 responses to “Signal Series – ABS, Part 1

  1. Pingback: Tweets that mention Signal Series – ABS, Part 1 « MAX FAQs --

  2. Wow!
    I really love your photos and easy to understand explainations!
    I was a little fuzzy on what “lunar” exactly meant; now I know. {Thanks}!

  3. I saw a TriMet vehicle on the road the other day and I was curious as to the purpose of it. It said “Signal Relay Test Van.” Obviously, it tests “signal relay” but what exactly is that? I have an idea as to what it could mean but I don’t know how to explain it. Maybe you could explain it?

    • Sorry, realized I didn’t respond to this yet! I don’t know a lot of the details because that’s a maintenance of way thing, not operations. All I really know about it is that the MOW workers can take the relays out & test them in the van, but I don’t really know what that entails or what they look for when they do it. I think we need a MOWFAQs blog :)

  4. FYI- Most of the system utilizes relays. The relays need to be tested depending on the type between yearly to once every four years. The relays are vital and ensure the safe movements of trains. Tri-Met has two vehicles dedicated to testing the thousands of relays in use on the system.

    • Can you or Camelopardalis explain what relays are in terms of signals (specific to the ones TriMet uses)? Thanks!

      • I know it’s part of the electronics system but don’t really understand how they work either – I understand that a train will be detected in a circuit, for example and in turn that can change signals, raise crossing gates, etc, but I don’t know the mechanics behind HOW it works. I’ll defer to signal_designer (or any other signal tech who finds themselves here).

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