Tag Archives: ats

Old OldTown/Chinatown signal

I forgot I had this picture, otherwise I would’ve included it as a point of sort of historical interest in my ABS/Pre-empt Combination Signal post:

Old view eastbound at Oldtown/ChinatownCab view, eastbound at Old Town/Chinatown several years ago

This is an old picture, taken back before the Portland Transit Mall was integrated to have the the Yellow and Green Line trains running on 5th and 6th.  That pre-empt signal isn’t there anymore – the way this used to work was that an eastbound train at Old Town/Chinatown would call their pre-empt and then begin to proceed up the Steel Bridge on a proper signal.  Signal 10 (which I have no picture of) was located on the bridge prior to the span, displayed a red or a lunar, and was associated with an ATS magnet – so if, for example, the bridge was going to be lifted, signal 10 would be red and the magnet would be active.

But then the Portland Transit Mall happened, and that meant tracks approaching the bridge from a different angle where the Yellow and Green trains go across the river from Union Station and now the added possibility of a Yellow or Green train making a conflicting move to a train at Old Town/Chinatown heading east.  So now Signal 10 is a combination signal located where this pre-empt was, and Old Town/Chinatown has an ATS magnet. I forget specifically when this change happened – in 2008 I think.

New signal 10The new Signal 10 – here the train operator has called it, but doesn’t have pre-emption of the intersection yet.

So there’s a little bit of TriMet rail history for you.

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!

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.