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:
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.
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.
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 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!
Coming 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.