This incident and the one about checking citizenship on the San Diego trolley have a common theme. There is a pretty good website all about internal border checkpoints:
Checkpoint USA.
The owner of this website is a pretty famous guy who has documented his encounters with the Border Patrol and other police agencies at a fixed checkpoint he apparently traverses daily to & from work. He has posted over a dozen YouTube and other videos about his encounters. It's fascinating stuff and, if you love our Constitution, downright scary. The guy has fought and won two separate lawsuits. (He's 2-for-2).
In addition to ports of entry -- land borders, airports, seaports, etc, I learned from some research on his site and referenced links that there are two types of internal border checkpoints permitted: fixed and roving. Each have their own set of rules and legal constraints.
It sounds as if a bunch of constraints about roving checkpoints were violated here. I would certainly check it out. As for the trolley harassment, I'm not clear whether this was a fixed or a roving checkpoint. If it's a roving checkpoint, they violated a whole bunch of constraints. If it was a fixed site, I'm not sure it fits that definition, either.
This could just as easily been set up in downtown Detroit as it was in San Diego. If this was something that the BP and TSA cooked up on their own, DHS has once again rubbed our noses in the Constitution.