Now that we have established not to drive around Germany and Italy in a car, would it be better to get train passes ahead of time in the U.S. or just get them when we arrive there? There are five of us. Are there different trains between cities and how often do they run? We will be going from Frankfurt Germany to Munich, then to Venice, then to Rome.
What I do is check schedules on this side of the Atlantic, and buy them when I'm in the EU--mainly because if a meeting goes longer or shorter than planned, I always have flexibility and not rushing to a station.
The only reason to get tickets ahead of time in the US is to get discount fares offered on the national rail websites. Other than that, get them in Europe.
For instance, from Frankfurt to Munich, you can get SparPreis fares as low as €29, purchasing as much as 3 months in advance. Note that these are non-refundable, date and train specific fares, not suitable for the day you arrive.
For the day of arrival, a group ticket to Kahl, the first station in Bavaria, and a Bayern-Ticket from there, although slower using regional trains, is the best deal as far as price.
Then, from Munich to Rome, you can get advance purchase Europa-Special Italien fares from Munich to Venice for as low as €39 pP.