On GWE I've just seen a 6-coach HST. Is this normal? Does this have anything to do with the PC-only services (that can be played on PS4 apparently and run fine)?
Yes. Journey mode uses the PC timetable. Starting journey mode then coming out and running services keeps the route in PC mode until you completely exit to the main menu out of GWE. Hopefully there is a new timetable coming with PC and console allignment.
