There appears to be a regular night train going from Sofija to Thessaloniki that leaves at 11:00 PM and arrives in Greece at 7:13 AM. I found this by using the Swiss timetable Website at I always have been able to find the schedule I am looking for there.

As for your trip to Hallstatt. The best route and travel time (5 hours) appears to be the 7:30 AM train out of Innsbruck with one stop in Wels. It takes a little effort to get to Hallstatt, so I would suggest getting an early start. Don't forget that the train station is on the other side of the lake from the town and the ferry across does not wait long at the dock. Have a good trip.

Greg