Made a quick test, and had no trouble placing the train using Free Roam but I can't get the train to move (keeps telling me to raise the pantograph when it is already up). Currently I think the Vorarlberg OHLE has a different voltage that the 801 can't use, but not 100% sure.
The power voltage isn’t compatible. If the pantograph is up and mcb closed and not working then it’s a different voltage line and won’t work.
The "free roam" mode is not the same as the "off the rails" mode you see on scenario planner. It means that the 25kV 50Hz power needed for the 801 is not supplied by the 15kV 16.7Hz overheads that Germany and Austria use. I guess it is one of the downsides, but we have scenario planner for that!
I wonder, can the 801 run on the Heathrow line between Paddington and Airport Junction? I have no idea whether those catenaries are functional or decorative; GWE is very old and designed as a diesel route.
If the power voltage isn’t compatible try switching over to diesel power. Start the diesel engine blue button next to two phones hold it til it has a lit blue ring around it then press yellow diesel button next to it to switch over currently driving an 801 from Huddersfield to Manchester Victoria NTP route.