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