I have a Swift and I use an 18x42 duffel bag to store the bike in my trunk (wheel and seatpost removed). I can tell you that the Swift website is correct; the bike will definitely fit into such a bag (albeit with a fair bit of cajoling), but the end result is HUGE. Seriously, it takes up more than half the space of my 16 cu.ft. trunk. On the bus? Good luck! Maybe if you told the driver it's actually a cello...
The Swift is a small bike that was designed to fold quickly into something shorter (but not necessarily smaller), such that it can easily fit into a closet or leaned against the wall out of the way (the narrow-ness of the bike shines here especially). On a train, it would still need to go to the bike or luggage car, and on the bus...well, it was designed with bike messengers in mind so not really optimized for that mode of travel. Note that the Swift's wheelbase is the same length as that of full-size bikes so using the bike rack on the front of the bus should be no problem.
If you absolutely have to bring your bike on board a bus, I would stay away from the Montague and Swift and instead look at the Brompton (or possibly the Tikit, but that's already getting a little big).