Here's a good website: Travel By Train In Europe: Eurorail, Eurail Passes & Train Tickets from Rail Europe

If you type in Paris to Mainz it tells you to contact them for help. But if you type in Frankfurt as your destination if brings up a bunch of possibilities. I took that 10:01 train and missed the 13 minute connection, but no problem, there are a lot of trains going to FRA from there.