There is no way of getting on to the platform other than by someone getting off releasing the gate for you and you then getting on before the train doors close!I don't think it's "set down only"; maybe it once was but the timetable for 2N99 above shows both pick up and set down. However I'm not local and I could be wrong!
Nothing is booked to stop on the down fast at Bushey, but of course diversions occur, as with 2N99 in this case.