Call loops

Question: Sometimes when the train stops at a platform, it suddenly jerks forward another foot or so before opening the doors.  Why?

This is an answer in several parts that requires an explanation of train-to-wayside communications.

First, the wayside part.  This is what’s known as a call loop:

Call LoopA call loop (Merlo/158th westbound)

Lloyd Center call loop…and what a call loop looks like when the rail is in pavement.  I forget where I took this, but I think it’s the Lloyd Center eastbound platform

These are located at most platforms on the alignment, as well as at several intersections away from platforms. Hold that thought for now.

Next, the train part.  I’ve never taken any pictures while I was underneath trains in the shop, but there’s a small, sort of wedge shaped device underneath the cab of each train called a transponder (the linked example is not identical to the model that TriMet uses, but it’s close enough).

When that transponder under the train passes over a call loop, it lights up four buttons inside the cab (which are referred to as “Vetag”, sometimes spelled “V-Tag”, which is short for “Vehicle Tagging System”) that the operator can use to communicate with the signals and switches on the alignment.

Vetag console (dark)Vetag, dark (picture taken from coupled end of trailing car)

Vetag lit daytimeVetag, lit (“Call” button not pictured, Old Town/Chinatown platform service)


Vetag lit nightVetag, lit at night (bottom left corner) to better show lighting, Beaverton Creek platform service

The first two buttons on that panel (“L” and “R”) are used in the railyards, the third button is used to cancel a command that’s been pressed, and the fourth button which says “Call” is the one most frequently used.  This, by the way, is one of those weird language things of rail – to press that button for a signal can be referred to “calling a signal”, but the word “call” in this context is different from the word “call” when used to refer to communicating with Control over the radio (see also: Call Boards). If you scan the radio, you may also hear this referred to as “selecting a signal”.  Anyway, when the transponder is over a call loop and the Vetag is lit, the operator can hit the call button to get their signal and throw power switches from the cab of the train.

VT on call loopCall loop at Rose Quarter eastbound (VT = Vintage Trolley)

So that’s an explanation of the basics of how a rail operator is able to move the train along the alignment.  Now to answer the “Why does the train sometimes move again after stopping at a platform?” question.

The call “loop” is actually more of a “call figure 8”  I’ve never seen the inside of one, but the circuit inside is kind of shaped like this:Loop circuit

The transponder under the train needs to be over one of the two loops formed by the circuit in order for the control panel inside the cab to light up.  If the transponder is over the spot where the lines of the 8 cross, the control panel goes dark and is unresponsive.  This is known as the “dead spot.”

Picture from MrK (see comments) – LAMTA, not MAX, but same idea 

So what happens when the train stops and then goes forward again at a platform is that the operator managed to stop the train with the transponder directly over the dead spot.  You cannot back a train up on the mainline, so their only option is to go forward – but they can’t go forward too far or they’ll overshoot the call loop completely since they’re already halfway over it.  So they move forward just enough to get the Vetag lit and then immediately brake – which, yeah, can be a bit jarring for a passenger who is expecting the doors to be opening, not to be moving again!

Call loop, OCC WestboundComing in to the platform at Oregon Convention Center, westbound – you can see the call loop between the rails as it passes under the train. Operators have to be able to stop on the loop such that the transponder under their cab is over the right spot of the loop to make their selection.

And since this only happens at platforms with call loops, you won’t get that jerking forward at (most of) the platforms on Burnside, since those don’t use call loops.

13 responses to “Call loops

  1. Pingback: Tweets that mention When the #trimet MAX train stops & jerks forward before opening the doors, this is why: --

  2. nope not gonna do it

    Actually, the “VT” on that loop is the indication for where the Vintage Trolley is supposed to berth. Since we don’t run the VT in revenue service at that location any more, it may not be as well known.

  3. nope not gonna do it

    Also… take a quick trip in the wayback machine with me: There used to be another reason for jerky stops. For double secret probation bonus points, what was it? This would be a neat research project for a next post on your blog. I’ll check in over the next few days and see if you know (and let you know if you don’t.)

    • Hmm, my first guess would be to line up with the wheelchair lifts.

      If that’s not right, I remember reading something (and I’m probably going to not recall this entirely correctly because it’s been a while since I read this so correct me!) where a 2-car consist used to be set up such that enough power or equivalent power or something had to be drawn in from both pantographs or the train wouldn’t move forward, and it was this sort of thing that prevented a train from rolling backwards slightly when the brakes were released on a hill (e.g. Pioneer Square WB). But then there was some issue with ice being on the overhead and that feature was some kind of an electrical hazard, so it was disabled and that’s why you get that rollback. So my second guess would be that it’s related to that, but I probably relayed that wrong.

  4. OK, I could have sworn you mentioned this somewhere before, but I’ve looked all over and I can’t find it.

    How do the switches know what position to set to? Like, if you are operating a Red Line train at Gateway southbound, how does it know to send you around the fishhook as opposed to sending you to Gresham? What happens if you need to divert a broken-down train into a pocket track?

    • I don’t think I ever really went into it, but that’s a good idea for a post. The short answer is that every possible destination on the alignment has a two digit number assigned to it called a route code. All an operator has to do is put in the route code for wherever they’re heading, and at every call loop when they hit the “call” button, the signals/switches will be set to get the train from where it is to that destination.

      Edit: I now do have a post on how that works.

  5. I love all this stuff! Have been reading up on all kinds of posts here :)
    I’d always wondered why this happened, but figured it had to do with something along the lines of lining up better with the platform. Good to know! Now I can explain it to my friends when they complain about the MAX doing this, and sound smart ;)

  6. As for the inside of the call loop cover….here it is….well sort of. This photo is from the LAMTA system I took several years ago. San Diego shares the same loop setup as this and I can only imagine that under the covers here may rest the same loop setup as the same shape is cut into the surface running areas. All PVC so it is RF inert so the wires inside the PVC pipes will pick up the signals with ease.

  7. Looks like the image didn’t make it through. Would be interesting to see this.

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