There are some commuters who use folding bicycles. Fairly light they fold in a few seconds into a large shoulder bag. Give that he bike itself deducts from your total cargo capacity your going to be limited in what you can carry. That said if your folding the bike to get on a bus it isn't like you're man hauling both bike and pack up a steep hill. Once on the bus your energy expenditure is limited. Pretty much if you can manage to trundle into the bus in good time and good order your good.