Also, thanks to the floods, the trains heading West out of Vienna are a bit of a mess. You will have to research and figure out if it affects the train to Hallstatt. Dont know. Have heard that the mess will be in place to some degree for up to 6 months.
See here how is dealt with this unprecedented situation for the time being:
https://community.ricksteves.com/travel-forum/austria/flooding-in-austria#2366395
Additonally, as every year, on December 15, 2024 there is the coordinated train schedule change of all European train operators.
All current timetable information past this date is tentative as long as the new schedule has not been confirmed (usually during November).
Therefore you will have to wait for concise information how to get to Hallstatt by train. But I don't expect big problems except a longer travel time.