Signal Series – ABS/Pre-empt Combination Signals

Okay, back to signals.  So there are your ABS signals, which indicate both track occupancy and switch position (by showing which route you are going on)

And then there are the pre-empt signals, which permit trains to go through intersections where the trains and cars run in mixed traffic.

But since pre-empts on their own don’t tell you anything about how switches are set or train occupancy, when in pre-empt territory you need…

ABS/Pre-empt combination signals

Which I admit I have a tendency to short to “combination signals” and then even from that to “combo signals” because that’s altogether kind of a mouthful.

Signal W2, a combination signal located at the 11th Ave Terminus

Combination signals can display both the red aspect of an ABS signal, indicating STOP as well as the yellow horizontal and white vertical aspects of a pre-empt signal.  They are used in pre-empt territory for several reasons:

1. To switch a train to a secondary or tertiary route

2. To indicate occupancy of someone else in the circuit

3. To prevent a train from moving into a conflicting move

4. To keep trains from going on the Steel Bridge if/when a bridge lift is in progress

Diverging to another route

Remember how the number of aspects lit on an ABS signal tells you which route you’re going on? (one aspect = primary route; two aspects = secondary route; 3 aspects = tertiary route, etc).  Well it’s the same with ABS/pre-empt combo signals.  I’ll keep using W2 as an example. If it helps, here is the 11th Ave terminus from above. W2 is located on the corner of 11th and Morrison and is used by trains heading west.

For Blue (westbound to Hillsboro) and Red (westbound to Beaverton Transit Center) trains under normal operating conditions, W2 will pretty much behave as a regular pre-empt signal once the operator places their call for W2 at Galleria.

Red on W2Red on W2

This indicates STOP and is what W2 looks like by default until an operator selects their route from the Galleria platform. Similar to a red on an ABS signal, reds on combination signals will be associated with ATS magnets that a train cannot move over.

Yellow horizontal W2Yellow Horizontal on W2

When an operator of a Blue or Red line train is at Galleria, they watch W2 (which is a block away) and wait for the red to become this yellow horizontal. That shows that their switches are set to continue straight on to Hillsboro/Beaverton (primary route, so one aspect), and by the time they get up to the intersection of SW 11th & Morrison, it (should!) turn to this:

White vertical on W2White vertical on W2

Which, as you already know, indicates “proceed with caution.” Because you get dumbass cyclists like that one biking against the light.  Good job, unknown cyclist – lucky for you that train was going straight and not going to hit you as you cycled across the diverging tracks!

11th ave switchesSwitches into 11th Ave Terminus

Here’s how the switches look to an operator – the train I was on when I took this photo was a Blue Line stopped at Morrison. This train would be continuing west instead of diverging into the terminus, and so the switches are set for the primary route, which is to head west towards Beaverton/Hillsboro. You can see the yellow horizontal on W2 in the top left corner of the picture, which indicates that the switches are set for the primary route but we can’t enter the intersection yet.

When Yellow Line trains used to turn around at 11th Ave, they’d get a different aspect after selecting their signal from Galleria:

Red over yellow horizontalRed over yellow horizontal on W2

The indication of this signal is “STOP – switches are set for something other than the primary route (more than one aspect), but you don’t have pre-emption to enter the intersection yet” – these aspects would display for trains diverging into the terminus after their switches were set and before the permissive white pre-empt aspect(s) came up.

So my picture of W2 showing a red over white diagonal over white vertical aspect indicates permission to proceed with caution on the tertiary route.  3 aspects  = tertiary route. The tertiary route from W2 is the easternmost diverging track into the terminus.

A secondary route on W2 would be the red aspect over just a white vertical, but I never got a picture of that – if all tracks at 11th Ave were empty, a Yellow Line train would first be put in the tertiary track, and the secondary track would be used if another Yellow Line train entered the terminus while the first was still there. That didn’t happen while I was standing there taking pictures that day.

Other combination signals that will show these same indications for primary, secondary, and tertiary routes can be found at end of the line signals W1760 immediately prior to the terminus at Hatfield in Hillsboro, and M164 which is the combo signal immediately prior to the Jackson turnaround for Yellow/Green Line trains at PSU.  There is also 18B into Rose Quarter from the east, which allows a train to continue on the normal westbound track (primary route), the special events track (secondary route), or the Vintage Trolley barn (tertiary route).

Secondary route, W1760Sorry for the blur but it’s the only picture that I have of an ABS/pre-empt combination signal displaying a secondary route – Secondary route on W1760 (under the car traffic light, it’s a red over a white vertical)

Circuit occupancy

On the other side of the 11th Ave terminus for eastbound trains is signal W6, another combo signal. An eastbound train coming up Yamhill can’t enter the terminus so W6 isn’t there for route selection, but a red on it will indicate that something else is in the circuit – either a train leaving the terminus, or a streetcar on 11th Avenue.

Red on W6 – because a train is leaving the terminus, so the ATS magnet associated with W6 will prevent an eastbound train from colliding with it

Permissive white vertical on W6 & eastbound train – you can see that the train that was pulling out of the terminus in the picture above this one is far enough ahead that it’s safe for this eastbound train to keep going.

Preventing a train from moving into another train’s conflicting move

There is a siding track that diverges off Holladay at 11th which used to be used by the Vintage Trolley and is occasionally used by MAX trains taken out of service (and yes, another combination signal – 20A – is used to make that diverging move).  Signal 20C at Lloyd Center westbound is associated with that siding track.

20C20C, Lloyd Center westbound

A train facing this direction has no option to choose a different route, but the combo signal will prevent a westbound train from moving forward if a train is going to go into or come out of the Doubletree Siding.

Steel Bridge Lift

All signals leading to the Steel Bridge (on the east side of the river that’s the Rose Quarter platform for Blue, Red, and Green trains and Interstate Rose Quarter for Yellow trains; and on the west side of the river that’s the Oldtown/Chinatown platform for Blue and Red trains, and the intersection of NE 3rd & Glisan for Yellow and Green trains) are combos. Whenever the bridge is lifted, it throws up reds on all of those surrounding signals.  As mentioned in the Automatic Train Stop post, although a yellow horizontal and a red both indicate STOP, a train still can physically move on a yellow horizontal, but it (by default) can’t move on a red.  So as soon as the bridge span is unlocked for a lift, all trains will be prevented from getting anywhere near it.

Red on M26Red on M26, the last mall signal before the Steel Bridge at 3rd & Glisan,  taken from trailing car of  a Yellow Line train

The interlocking of tracks around the Steel Bridge is extremely complex – on the west side, the Yellow and Green Lines cross the Steel Bridge from a different angle than the Red and Blue Lines, and then on the east side the Yellow Line turns north towards Expo and the other lines continue east.  Since all of those moves are done through switches in the rails, the combination signals will also display reds if another train is making a conflicting move (e.g. if a PSU-bound Yellow Line at Interstate Rose Quarter leaves the platform just before a westbound Red/Blue/Green line train tries to leave Rose Quarter), or if the switches aren’t set right for you – this is why sometimes you will be sitting on a Yellow or Green line train at 3rd & Glisan waiting to cross the bridge for a while (which means a Blue or Red left Oldtown/Chinatown and got onto the bridge before you got there), or similarly be sitting at Rose Quarter on a Blue, Red, or Green train waiting for a Yellow Line to get out of the way.

Reds on 16B and 16CReds on 16B (special events track at Rose Quarter) and 16C (main westbound track at Rose Quarter)

Both of those above ABS/pre-empt combination signals are capable of:

switching a train to another route (this is how trains can get from here over to the Expo Center from the Ruby Junction railyard – they can diverge from here)

preventing a train from moving if there is another train in the circuit

preventing a train from moving if the switches aren’t set right for them, and

stopping a train if the Steel Bridge is going to be lifted.

And that pretty much covers the last of the 3 major signal types used on the MAX light rail alignment.

10 responses to “Signal Series – ABS/Pre-empt Combination Signals

  1. I was hoping/wondering if you could possibly please explain the last signal before Hatfield..I think it’s a combination signal but I haven’t been out there for a while.

    I always get confused when I try to think about how that signal works, so I’d really appreciate it if you could explain how that works. :)

    Also, do you know how it’s possible that the TWC code for the Main Street Pocket Track is the same as the TWC code for Gateway – EB Main (From PDX)?

    Matt :)

    • Sorry for the delay, it’s been a busy week for me! :)

      Okay, in two parts:

      1. That last signal before Hatfield is W1760, which is a combination signal, and it shows the same aspects as W2 (I’d just spent more time taking pictures of W2 than W1760 so that’s why I used W2 as my example in this post). There are three tracks in the Hatfield terminus – the primary and secondary tracks, and then the auxiliary track (which is the tertiary route). Most trains going to Hatfield use a route code that means “either open track at Hatfield”, i.e. the primary or secondary track. Some trains are scheduled to pull into the auxiliary track so they’ll be using a different route code.
      Anyway, an operator will place their call for Hatfield when they’re at Hillsboro Central. If they’re aiming for either open track, W1760 will display a white vertical if the primary track is open, or a red over a white vertical if the secondary is open, which is the blurry picture I posted here. If they were aiming for the auxiliary track, it will display a red over a white diagonal over a white vertical.
      The only difference between W2 and W1760 is that the red aspect is round on W2, and it’s a horizontal bar on W1760, but a red is a red is a red, doesn’t matter what the shape is it still means the same thing.

      2. They can be the same because they’re used by trains in different places. A train at Main Street is in an entirely different circuit and location than a train at Parkrose, so the route code is going to throw the switches for what is immediately in front of them. In sort of the same way, the route codes for the storage tracks in the yards are the same, but you won’t end up at, say, the 6th track of the Elmonica yard if your train is in the Ruby yard and you’re trying to get to storage track 6 there.

  2. nope not gonna do it

    Re: 20C – does not protect the 15 crossover (the manual hand-throw switch between 7th Avenue and Lloyd Center). Manual hand-throw switches, even in girder rail territory, are not tied into the signal system. Remember, you can trail a mainline girder rail switch when set against you – therefore nothing to worry about stopping trains. A move out of the trolley siding is a whole nother story.

    • You know, I didn’t think to check my alignment book if that was manual or power and I’ve been working under the assumption that you can’t tell by looking at a girder rail switch since it lacks indicators, and therefore have largely been making the assumption that most of the girder rail switches are power (except for the ones that I knew are not).

      Though now I am also thinking you should take this over from me!

      • nope not gonna do it

        No – you’re doing a fine job. I just like filling in little details here and there.

        • Well thanks! And I like having the details filled in (and inaccuracies corrected), even though I’m not like a spokesperson or anything like that and this isn’t an official TriMet publication by a long shot, but I’d prefer to have good information up at least!

  3. I just noticed something (after re-reading this post for my school project)…

    One of the pictures above shows signal 20C but has a caption saying it’s at Lloyd Center Eastbound…20C is Lloyd Center Westbound :)


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