A nice string to debate has formed in the comments from my previous post about whether one can straight up take an aisle seat when both are available? And what is the courtesy?
I don't impose any rules or etiquette on anyone, they can take whatever seat makes them happy. BART is truly first come first serve. Just like on Southwest Airlines, when you are in Boarding Group A, most people take the aisle seats even though it slows down the rest of the boarding process since you have to get up and down for passengers who come in later. HOWEVER...I do think people who take the aisle seat MUST get up when someone is trying to get into the window seat. And NO, just scooting your legs to the side, or crossing your legs to make room, is not good enough because I still need to crawl over you. As long as someone is willing to get up whenever the window seat is being taken or vacated, I'm fine with them taking the aisle seat.
Personally, I always slide into the window seat on BART except when sitting in one of the four-seat areas where it's not as tight for people to walk in and out of. In fact, an aisle seat in one of the quad-seat areas is generally my #1 go-to seat whenever it's open!