Train travel, Rome to Venice to Innsbruck to Vienna to Brno to Prague

You only need a supplement if you have a Eurail pass which you don't seem to need for this part of your journey. No pass - no supplement. If you do have a railpass, the supplement will be 10E per person. There are several high-speeds a day from Rome to Venice. You can easily buy the train tickets when you arrive. The trains don't run full. You can try buying from www.trenitalia.com but people with U.S. credit cards have trouble making a purchase. The 2nd class fare is 72E ($100) each for a one-way journey. If you are traveling to, and will visit Vienna, Brno and Prague, you will need to buy point-to-point tickets for each journey. Some trains on the journeys (you often will need to change trains) will require seat reservations which you can make when you buy the tickets. You can buy your ticket from Venice to Vienna at the Venice train station or you can buy on-line at www.oebb.at. This is the Austrian train website. Others should be able to assist with the other tickets and where to buy them. You do have a long journey to Prague. You can see all of these journeys on www.bahn.de. This is the German train website. You can't buy the tickets as the triip does not involve Germany but you can see the journeys. Note that on this site, even if you select English, Venice is Venise and Prague is Praha.

Daytime connections from Venice to Vienna consist of an InterCity bus run by Austrian Rail from Venice to Villach, then train from Villach to Vienna. If you book online in advance (I'm not sure how far is permitted, but apparently not 92 days like with the Bahn), you can get a Sparschiene ticket for as low as €29 per person, Venise to Wien. There are also all rail, overnight connections. Looks like a full fare ticket (2nd cl.) from Vienna to Brno is €25,60 while a Sparschiene ticket is €19. Considering that the Sparscheine ticket is advance purchase, train specific, non-refundable, it might not be worth the savings.

Thanks Lee. If I buy the 5 day 3 country pass from Rick Steves I believe I can buy the supplement to go high speed from Rom to Venice from what Larry is saying. I don't know if they have high speed with the other destinations.

Thanks Larry. I don't know if you got my other message.
This helps a lot.

I missed the part about Innsburck. I'm sure if you buy p2p tickets, Rome to Venice, Venice to Innsbruck, Innsbruck to Vienna, Vienna to Brno, and Brno to Prague, it will cost less than a rail pass with surcharges.