Train Question

Its a little more complicated than that as there is no train station at the airport. Right now you can take a bus to the closest Metro station and take that to Prague Main Station for trains to Vienna. I don't like prebuying tickets on day of arrival unless there is a huge discount as an incentive to take the chance that your plane arrives late. And a train purchased in CZK will be pretty cheap to start with.

Wait for Unclegus or Marcus Bradshaw to chime in.

I think there's a direct bus from the airport to the train station - probably easier than the bus-Metro-train station connection. Check Rome2Rio.

Looks like there is a train from Prague Hlavni Station to Vienna Hauptbahnof Station departing at 12:51 and 14:51. Is it a matter of taking a train from the airport to Hlavni Station? Is 12:51 too optimistic with an 11:00 arrival?

Yes because as you know from flying or hearing about delayed flights that an on time arrival is not guaranteed. But buy the 12:51 the day off if you see that you can make it 30 minutes before the train leaves

Besides the Railjet direct trains, which have a walk up price of 35 EUR equivalent per person, there is the new Regiojet direct train service costing 15 EUR equivalent per person. Afternoon trains at 13:21 and 16:21.
https://www.regiojet.com/

http://www.pragueairport.co.uk/airport-express-bus/
there is a bus direct from the airport to the train station, I have linked the webpage.
takes about 40 -50minutes depending on traffic and they run every 30 minutes, bus is right outside the terminal.
cost is 60kc and you can pay the driver, but don't use ;large notes as they are unlikely to have change,

Thanks everyone. That is very helpful information.