Could also depend on when the allegation of rape was made and the condition of the girl at the time (traumatised, possibly intoxicated - who knows with young girls being out getting buses home alone that late at night).
I agree with ummmm about the freezing aspect and given there were too men she was more than overpowered, possibly gagged by one.
If the First double-deckers are like the ones here, the seating is quite dense (not sure if that's the right word) on the top of a double-decker.
I've been on many a bus when it sounds like all hell is breaking loose upstairs and the driver just carries on - often on school run buses (which I avoid if at all possible now) but I've seen some nasty fights too, one of which involving a number of men with metal bars and one ended with two men fighting off the bus and both going through a glass shop window.
People seem to be too afraid to intervene, especially in what seems to be violence and if they were being loud and abusive it may have led to anything else which might have happened being misunderstood.