Train Prague to Vienna

Whether you have to change trains is easy. Go back to the site you booked the tickets on (or use www.bahn.de ) and look up the train again. It should show you the number of changes. There will be an icon to expand and this will show where and when to change. If you click on the train number it shows all the intermediate stops for that train.

As for platform numbers. It also shows that if known. But these are always subject to change at short notice, so you should always check the station departure board.

Without any information on departure and arrival time, it is impossible for any of us to say. Go to www.bahn.com and match up the information to see for yourself.
Regarding track numbers, just like airline gates, they change as circumstances dictate. Just look at the departure board when you get to the station.

Hopefully you bought a ticket for a direct train as it is an hour faster. You'll find the platform number when you arrive at the station. Why do you need to know the platform now?

@ Emily - see "Chronic research -over thinking syndrome" thread! 😉