Monthly Archives: April 2010

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.

I lied – More signals!

Okay, I forgot two.

Dwarf Signals

Ruby Jct/E 197th, looking eastDwarf signal in lower left corner

I love this picture because there’s so much going on in it.  Ruby Jct/E 197th looking east, zoomed in (I took the picture from the westbound platform) so the quality is a little grainy and the distance is flattened out – objects in the picture are much farther away than they appear!

I forgot the dwarf signals because under normal operating conditions, you don’t really see them because they don’t face you. Dwarf signals for MAX are only used when running reverse – notice how in the above picture which is facing east, the dwarf signal is on the westbound track, so you’re only going to see it if you’re going east in the westbound.

Anyway, the purpose of dwarf signals are to protect power switches on the mainline when you’re running reverse in ABS territory. They won’t display any aspect other than red, though some will go dark if there’s no train in the circuit. They’ll be associated with ATS magnets and switches.  When running reverse without signal protection, you must stop at all switches to make sure they are properly set since you’ll be coming at them from the wrong direction.

Dwarf signal at BTC“Dwarf” not always short

Here’s dwarf signal W768 and two standard ABS signals at Beaverton Transit Center looking east.  They’re all the same height, but the dwarf is the only one that can only display a red aspect.  In this picture it’s dark because there’s no train in the circuit.

Same signal, now with a train in the circuit (in the westbound platform)

That particular signal protects the switches that Red Line trains take into the center pocket track at Beaverton Transit Center – which are located pretty far around that curve, so dwarf signals aren’t always right on top of associated switches.

The C Signal

For lack of a better name, anyway.


There’s only one of these that’s still in use, and it’s at Skidmore Fountain westbound (there is also one at 11th & Yamhill, but it is not active). The intersection after this platform is SW Ash, but you can’t see it from here, so when an operator selects at Skidmore westbound, the “C” signal illuminates to let them know that the call went through and they can leave the platform.  When they get to Ash, they should have their pre-empt.

Next up – not sure yet, but it won’t be signals!

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.

Automatic Train Stop

A pause in the signal series, because it was getting too difficult to write about the last type without having explained ATS first.

ATS, or Automatic Train Stop, or “What keeps the trains from crashing into each other?”

Link to Wikipedia article, for those that like that sort of explanation

ATS in t-railATS magnet.  I don’t remember where I took this pic, but I think it’s Beaverton Transit Center, westbound platform

ATS in girder railATS in girder rail, Lloyd Center westbound platform

These little yellow rectangles are found all throughout the alignment, much to the delight and happiness of rail operators.  These are the ATS (automatic train stop) magnets, and given their name it’s pretty easy to figure out what they do. If a train goes over one while the magnet is active, the train automatically comes to a stop.

They are associated with every signal capable of displaying a red aspect, and will be active as long as that signal is red.  So, by default, a train physically cannot run a red light – attempting to do so will bring the train to an irretrievable stop.

ATS magnet and red signal aspect, Sunset TC westbound.

That magnet is currently active. Stopping the train and selecting for a permissive signal will turn the magnet off once that signal displays something other than a red aspect, which allows the operator to move the train forward again. If the operator had tried to keep going, her train would have been brought to a stop (and she’d have some explaining to do and paperwork to fill out!)  Again, if you scan the radio, that’s what’s called a “trip”, sometimes also referred to as “popping a red.”

ATS magnets are also located in areas where it is really unsafe for a train to speed – for example, coming into Gateway TC from any direction because of how busy it is, or the single track that goes into PDX International Airport.  These “speed trip” magnets have a pickup a set distance away from the magnet depending what the speed limit is, which activates the magnet for as long as it would take a train going faster than the posted speed limit to reach. So with a 15mph magnet, for example, if a train is doing 16mph when it goes over the pickup and doesn’t slow down, when it goes over the magnet it will trip it and come to a stop.  And again, the operator will have paperwork to fill out and explaining to do, because any type of ATS trip – from running a red or speeding – is a rule violation.

How a speed trip worksMore or less how ATS works for speed – let’s say that distance takes 30 seconds to cross if you’re going 15mph. If you operate a train through it and it only takes you 25 seconds, you’re speeding, and that magnet will stop you.

Although new operators especially don’t like the magnets since it’s hard to remember the speed limits of all parts of the alignment at first (and therefore easy to get tripped!), they are an extremely important safety feature and it’s a very good thing that they’re there – they prevent collisions in ABS territory (which covers all high speed areas) because a train will come to a stop at the red light, long before getting close enough to the train in front of it to hit it. And they prevent derailments or other accidents in areas where speeding would be extremely dangerous.

Signal Series – Pre-empt Signals

The second type of signal that is used on MAX alignment are pre-empt signals. These are the signals that pedestrians, cyclists, and car drivers are going to see most often.

What they look like and what they mean

A basic pre-empt signal has two aspects on it – a yellow horizontal on the top and a white vertical underneath (there is an exception to this rule at Kenton/Denver where the white vertical and yellow horizontal share the same top aspect, and I don’t know why that one is set up that way. I know that that pre-empt is tied into the ABS circuit of the Vanport Bridge so that might be related, but I’m not positive.  I’ve never gone over there as a destination (the Dancin’ Bare just isn’t my thing) so I don’t have any very good pictures of the signal, but you can sort of see it in this photo taken from the passenger area.  The quality isn’t great, but it shows the white vertical on the top part of the signal instead of the bottom. There are some pre-empt signals that are slightly more complicated and I’ll get to that the last entry of the signal series, but here are the basics first.

Yellow HorizontalYellow Horizontal pre-empt

To a train at an intersection, this is the equivalent of a red traffic light.  To put it in rail terminology, the aspect (what the signal is displaying) is a yellow horizontal, and the indication (what that means) is STOP.  A train cannot enter an intersection on a yellow horizontal without first getting permission from Rail Control and following Standard Operating Procedures to move through the intersection on a yellow horizontal (if you ever are scanning the radio and hear an operator ask to “SOP an intersection“, it means their signal is a solid yellow horizontal and they don’t have the ability to change it to a white vertical on their own to keep going)

White VerticalWhite Vertical pre-empt

And this is the equivalent of a green light for a train.  The aspect is a white vertical; the indication is “proceed with caution” – kind of like when you have a green light in a car, you still need to scan the intersection in case someone on the cross street is running their red light or a pedestrian is about to cross. This happens a lot with trains because people tend to watch traffic signals, not rail signals at these intersections so they don’t know that a train necessarily has a “go” signal.

(I made these next animated gifs myself. Don’t I have great photo editing skills?  You should have already known that from my ticket machine monster)

Flashing white verticalFlashing White Vertical pre-empt

This signal’s aspect is a flashing white vertical, and its indication is the equivalent of a yellow light for a car – it means that the white vertical (go) is about to time out to a yellow horizontal (stop) – so if the operator hasn’t yet entered the intersection, they should stop the train.  Some intersections time out more quickly than others – if you are sitting towards the front of the train on the right side heading west into Lloyd Center, the pre-empt at 13th begins to time out as the train enters the intersection and you can see that from the passenger area.  Similarly, the pre-empt at 4th & Yamhill which is on the left-hand side will also begin to time out as the train is in the intersection.

Flashing yellow horizontalFlashing Yellow Horizontal pre-empt

This aspect is a flashing yellow horizontal. There isn’t a direct equivalent of this signal’s indication to what a car driver would see on a regular traffic light.  The closest would be when you are stopped at a red light and you watch the color of the cross-traffic light, and when that turns red you know yours will turn green soon.  For a rail operator, a flashing yellow horizontal pre-empt means that it will soon become a white vertical.  At most platforms downtown, this means they need to have their doors closed (and not on release!) so that they can get going as soon as their white vertical comes up.  For intersections along the Yellow Line and on Burnside (and a couple of intersections downtown such as Skidmore Fountain eastbound and Lloyd Center eastbound where the intersection is not immediately in front of the platform), operators are permitted/expected to be moving the train up to the intersection on a flashing yellow horizontal indication, but they may not enter the intersection until it displays a solid white vertical.

Go, stop, goSo is this a go?  Or a stop?  Depends on if you are a train operator, a car driver, or a pedestrian!  SW 16th & Yamhill

Now here’s where it can get a little confusing – in the CBD (downtown), not all of the intersections have pre-empts.  At several intersections, the trains will follow the same red-yellow-green traffic signals that car traffic does.  However, every intersection where a green light would permit a car to make a left turn is pre-empted. This can cause confusion for someone who misses a train and then gets mad, complaining that the train left the platform on a red light.  Well, yes, they did – but the red light wasn’t a STOP for them, it was just a stop for the cars.

Pioneer eastbound white vertical Pioneer Square, Eastbound

Notice how the traffic light is red, the pre-empt immediately next to it is a white vertical, and the crosswalk signal (all the way to the left of the image) is a walk. That’s because at this intersection of 6th and Yamhill, cars are permitted to make a left turn from Yamhill onto 6th – over the train tracks -when that light is green.  So the red light holds the cars there (otherwise they’d be turning directly into the moving train!) and the train will go first on their pre-empt.  The parallel crosswalk is a walk sign because the train is going to block cross car traffic on 6th.

Pioneer Square westbound has the same setup:

Pioneer Square westboundPioneer Square Westbound

Again – red traffic light, walk sign on the crosswalk, and the white vertical pre-empt (it’s a little hard to spot – look between the two lights on the lamppost) – and obviously a train moving through the intersection!

Where you find pre-empt signals

Pre-empt signals are used in areas where the trains run in mixed traffic and intersections are protected by traffic lights, not crossing gates.

Map of pre-empt territoriesAreas in orange are pre-empt territories

Similar to the ABS map, the above map isn’t perfect because some platforms on the borders of ABS and pre-empt territories have an ABS signal in one direction but a pre-empt in the other, or both an ABS and a pre-empt in one direction, but it gives you an idea of where these signals are found. From west to east:

Hatfield Government Center to 12th/Washington

Out of the tunnel (there is a pre-empted intersection just west of the Goose Hollow platform) and all of the CBD, including the 5th/6th Ave transit mall and Yamhill & Morrison streets.

Over the Steel Bridge, the Yellow Line is pre-empted through Delta Park. The Blue/Red/Green Lines are pre-empted through 13th Ave, east of Lloyd Center.

The Blue Line is pre-empted from the intersections prior to E 102nd Ave on Burnside to the intersection just before the Ruby Junction/E 197th Ave eastbound platform.

Pre-empt signals on their own are pretty straightforward, so they only get one post.  Up next, what should be the last post about signals (at least for now), ABS/pre-empt combination signals!