the last time i did train travel in italy, we had 3 choices:
eurostar,(ES) high speed and expensive
inter city(IC) both first and second class. almost as fast as the es, and less than half the price.
locals(forgot the name), but very slow.(take about 4x the es or ic.)

look into IC's. bring food and wine. the trip from bologna to venice adds less than 30min to the trip. first class is usually empty. the seats are huge. great way to travel.

a number of the train stations require going down a long flight, and then back up to the train tracks.(venice does not).