...I just rewatched relevant bits of Quarantine (which I have to say is an ep I reeeeeeally dislike) and it doesn't make it any clearer. They're stuck and trying to pry the doors open, yes, but it's also at an actual stop-point, rather than somewhere in the middle, so it looks like they'd just got on (or just arrived) when the lock-down happened and the actual method of transport is still up in the air...