Brussels is too far to do by train in one day. It takes over 20 hours and there are very few overnight trains. That distance is best done by air.
Brussels airlines flies from Brussels to Venice. Website: https://www.brusselsairlines.com/ They are currently offering that route for:
Wed, 20 Dec € 50.18
Thu, 21 Dec € 75.18
Fri, 22 Dec from € 144.18
Sat, 23 Dec € 303.18
Sun, 24 Dec € 50.18
- As this is near Christmas, expect all transport to be booked up early, and expensive
- The annual train timetable change takes place on the second Sunday in December (10th December this year). Some railway companies have not published all their new times yet.
- I would not trust eurail, it has a reputation for only listing the trains it sells tickets for. I would stick to DB ( https://www.bahn.com/en/view/index.shtml ). But DB is not yet listing times after 10th December.