So I did a bit of research. In the off season the difference between train and riding is much smaller.
I used Michelin's route planner to work it out for me - Dusseldorf to Villach. A route I did back in 1986 bit from Cologne on that occasion.
The train comes out more expensive but does the trip overnight. To do that I'd have to tranquilise Claire "Mr T" style.
So motorail it is!