I've never had an issue on BART, but I also haven't tried taking it on during rush hour through the tunnel. I have a friend who has the crosstown who takes it on Caltrain every day and has never had an issue, so I'm guessing you're pretty safe.

The Jack is a great bike for a commuter, but it's definitely clunkier and heavier than the Boston. I also think the folding mechanism on the Jack makes the bike far less stable.