The Tauern tunnel (Between Bad Gastein and Mallnitz) will be closed from November 18, 2024 to July 13, 2025 due to construction work.
A rail replacement bus service will run between Bischofshofen and Spittal-Millstättersee. The journey time between Salzburg and Villach will be extended by approx. 30 minutes with 2 changes.
Flixbus may be the better option during this time.
The night trains (Nightjet) from Munich/Stuttgart to Rome/La Spezia/Venice are canceled without replacement.
Details are published by OeBB. I did find a Web page in German only.
https://www.oebb.at/de/fahrplan/baustelleninformation/tauernsperre-schwarzach-mallnitzobervellach?mtm_campaign=tauernsperre