Wonderful wealth of information. Many thanks to you all. Only 11 weeks to go before we leave. I'm off to tie up the loose ends and buy what train tickets I need.
Cheers.

The whole Westbahn itinerary Salzburg - Innsbruck is along the mountains without going into the mountains. In the case you would like to do the latter, go either
from Zell am See to Kitzsteinhorn,
or from Jenbach to Zillertal,
or from Innsbruck to Stubaital.