If you are leaving at any point from Frankfurt Airport, onto a train, know that there are two train stations there, one for regional trains only.

You will probably want the bigger train station which you must go down a very long hallway to reach. You will know it by its large glass dome overhead, but on the way there are many ticket brokers that advertise train tickets. Go past these and get your ticket directly from a train ticket booth in the glass dome area above the station. The train area is just down the elevators/ escaltors.