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