@jon i've never seen a bus where a normal person could get a bike on. A young gym nut could do it, but you need to to be rather strong to get a bike through a bus door and around the corners. This is about leverage and control, I could do it if given time on an empty bus. Howeve you need to not hit the driver or other passengers while the bus moves.
On a train it doesn't look that bad, but every metro i've been on (only that Canada line, so not a useful sample) has been packed.