it's in the working timetable, but that doesn't mean there's still track there.
Looking on OpenTrainTimes, I can't see any actual trains that have attempted the run lately, and various other sources have said that the track has been lifted because it's in the way of the HS2 construction. Apparently, there's passive provision for re-instating a link from Quainton to EWR. Survey on-site required. 
See https://www.youtube.com/watch?v=VziVb12Fris