The quick and easy way to get from the airport to Frankfurt's main train station, the…read moreRegionalbahnhof runs trains often enough that you're not going to be waiting too long for your ride.
Follow the signs to the Regionalbahnhof at the airport. If you're at Terminal 2, you'll need to take the free shuttle bus to Terminal 1 and then follow the signs to the escalator downstairs.
There, you'll find a ticket vending machines with optional languages including English. Easy to purchase a single pass into Frankfurt for an adult (I think it was €4.50) and the machine takes cash and cards. I had cash, so I don't know if it accepts US "swipe and sign" cards.
The trains run On. Time. here with no kidding. The posted schedule says 10:01, the doors close at 10:00 and 45 seconds then the train starts moving as the second hand sweeps the 12. No kidding.
Three stops on this line to the main station. Easy. Clean. Smooth. Much better than a taxi, I think.