Question: What do you mean by “circuit”, anyway?

I’ve mentioned rail circuits in a number of posts but I’ve never really explained them. So that’s what this post is for.

Don’t step on the rails

The rails are actually electric circuits – there is an electric current passing through the rails on all parts of the alignment. The voltage is low (15-25 volts) so it’s not harmful, but it’s just one of those “best practices” to not step directly on a rail. I’ve never heard of a person getting shocked by them, though there have been problems in the past of service dogs (see attachments 3 & 4 for TriMet specific examples) being shocked when stepping on the rails, particularly in wet conditions. As far as I know, that problem has been resolved and hasn’t happened in years.

Insulated Joints

The boundary between two circuits in the track is called the insulated joint. You won’t see these from the train, but you can feel/hear them as the train passes over them.

Insulated joint.  I forget where specifically this was taken.

I don’t have any video that I’ve taken myself which provide good audible examples of what it sounds like when a train is going over the insulated joints. Luckily, other Youtube videos provide. I’m shamelessly borrowing these examples – I did not shoot the video in either and take no credit for them.

  • Train at Old Town/Chinatown, video taken from outside the train (link should take you to the right spot at the video, but if not, move the seeker to about 7:11).
  • Train at Merlo Road, video taken from inside the train (at about 1:48)

What you’re listening for is that short sort of skipping sound made as the wheels pass over the joint. On the external example, you can hear it as each of the three wheel trucks on both cars passes over the joints (sounds like 6 sets of 4 clicks); inside the train only one truck is clearly audible.

Insulated joints in CBD – this might be Lloyd Center westbound, I don’t remember

The basic “how it works” principle is that the wheel axles of a train shunt the circuit in the rails, thus detecting the presence of a train in that particular circuit. As the train moves along the alignment, it passes over the insulated joints from one circuit to the next, which indicates what sections of the track are occupied by a train.

So speaking of track occupancy

As you might have guessed, insulated joints and track circuits are directly related to ABS signals.

Bringing up this picture again:

Train movement in the diagram is from left to right. To review, if you’re in ABS territory and you have a green, that means you’re clear for two ABS blocks, or the distance between the next two ABS signals. A yellow tells you that the block ahead of you is open, but there’s a train in the block in front of that. And if you get up to the next ABS signal and it’s red, that means the train in the block in front of you hasn’t left yet.

So how do the signals “know” that there’s a train there?

If a shunt is detected in the rail – meaning that something is going across both rails closing the circuit (namely the wheel axles of the train), the signal displays an aspect that indicates that the circuit is occupied – a red if it’s the circuit immediately after that signal, or a yellow if it’s not the circuit immediately after the signal but the circuit after the next signal.

So when you have a red ABS signal, that means a train is shunting the circuit after that signal and that section of track is occupied. Once that train ahead of you passes the next ABS signal, which will have an insulated joint associated with it, it leaves that circuit and the signal that you’re waiting at will display a permissive aspect. This indicates that the block in front of you is now open.

A good place to see how this works are the ABS signals at platforms. If you’re sitting near the front of the train on the left side, you can watch the signal change from yellow or green to red as you hear the wheels pass over the joint, which indicates that that circuit is now occupied by a train. If you’re sitting further back, the signal will already be red as you pass it. From outside the train, it looks like this:

Eastbound at Beaverton Transit Center

You can’t see it in the above picture, but there is a train immediately to my right that called signal W754 and got a green. Notice the insulated joint right about at the corner of the platform. Once the wheels pass over the insulated joint, shunting the circuit associated with that signal…

The signal becomes red, because now there is a train occupying the circuit. To conserve power, this particular signal will go dark once the circuit at the platform is no longer shunted, but if another train comes into the platform before this train passes the next ABS signal, W754 will display a red aspect again.

A blurry picture, but it was taken from the trailing parlor cab of a Type 4 – so the three wheel trucks of the leading car have already shunted the circuit after the signal, making it appear red for the rest of the train.

14 responses to “Circuits

  1. I’ve heard it before, but not sure where I heard it.

    Is it true that you can be electrocuted (whether it be a little bit or a lot) by standing on a v-tag?


    • Not that I’m aware of, and people walk over them pretty frequently (more often downtown where jaywalking across the tracks is common, but just last week I saw someone walk across the eastbound tracks at BTC and step on the call loop). However, since walking in the tracks is a dumb idea in its own regard, I’m willing to let people believe that stepping on the call loops will shock them if that means they’ll stop walking on tracks where they don’t belong because clearly the fear of being hit by a train isn’t doing it for them.

  2. There’s actually quite a lot of current in the rails when there’s a train nearby. All the traction current that the train gets from the catenary has to go back to the substation via the rails. But the rails are supposed to be at the same potential as the ground, so there should be no danger of electrocution from just stepping on one, assuming everything is working correctly. One case where there’s a very real danger, though, is when track work means that the rails have to be cut: then if the rail was the only path for the return current and there’s a train in the section, you might suddenly get 750 volts across that gap.

    • That reminds me of how (not that I’ve ever had to do this firsthand, which I’m thankful for) the first thing that a rail operator has to do in the event of a collision is drop the pantographs – even before calling Control to report the collision, because of the risk of electrocution to the person or vehicle that had been struck.

  3. Occasionally there is a “third rail” in between the tracks. I always assumed it had something to do with blocks and signaling – what’s that all about?

    • I’m not sure exactly what you’re referring to – if you meant non-TriMet systems, generally speaking, a third rail is used for power in place of the overhead wires. TriMet does not use this system since an electrified third rail puts the high voltage in reach of people should they access the tracks. Other systems that run trains of more than one gauge (width) will use a third rail so that both trains can fit on the same alignment, with the narrower trains making use of the inside rail. TriMet doesn’t do this either since all the MAX trains are built to the same gauge.

      If you were referring to seeing extra rail between the tracks on the MAX alignment, those are guard rails.

  4. I’ve been wondering, is there something that prevents the ATS magnets at platforms from stopping the train when the rear car passes over them while leaving the platform?

    • Yes, the sensor on the train is only on for the active cab (the one that the operator is keyed in to) – so when either of the coupled cabs or the trailing cab passes over the ATS, those sensors aren’t active so the train won’t stop.

  5. Thanks for clarifying that, makes a lot of sense now :)

    I love reading this blog and gathering every scrap of information I can from it!

  6. I have a related question (not covered in the blog elsewhere from what I could tell): do the automatic gates at the road crossings work on the same principles? I’ve always been amazed how the time between a gate closing and a train arrival is almost always the same, irrespective of the train’s speed. How is this achieved?
    Also, great blog! :)

    • Yes, it’s very similar. I wrote about this when discussing reverse traffic because that affects how the gates are lowered. Where crossing gates are immediately adjacent to the platform, the operator will use the TWC call loop and call them (with the exception of something like 12th & Washington which is mass-detected).

      For crossing gates that aren’t near a platform, there is a circuit called the approach circuit, and when that’s shunted the gates will lower. It’s a federal regulation that the gate arms must be fully lowered for 10 seconds before the train goes through the intersection, so the approach circuits for gated crossings are placed at a distance that allows for enough time for the gates to come down and be in that position for 10 seconds. It’s not exactly irrespective of the train’s speed – if I’m speeding, it’s possible to shunt that circuit and get to the crossing before the gates have been down long enough, so it’s still very important to adhere to the posted speed limits!

      If a train is running reverse traffic (e.g. east in the westbound track), there isn’t an approach circuit from that direction, so there is a smaller circuit called the island circuit that extends through the intersection, and when that is shunted the gates will come down.

      • Do the circuits associated with gates make the same clicking sound an insulated rail joint would? Or is it just something silent?

        • No, I don’t really notice anything. Of course, I tune out most of the insulated joints too, aside from a few that I got in the habit of using as markers for when to start braking.

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