Reminder: BART seats are for butts
A PSA: Just a reminder a BART seat is for a human body. Not: your feet, your bag, your other bag, your dry cleaning, your burrito.
Also: seats by the doors are for people who need them most. If you're seated on a busy train, look up from your phone periodically to see if someone might need to sit more than you do.
If a car is empty and you want to put your bag on the seat, great. But your stuff generally shouldn't be taking up a seat when people are standing.