Just remember the S-Bahn trains do not say "Frankfurt" on them, but rather the last stop on their route. (Somebody can chime in with the stop that will show). I once waited for an S-Bahn to Frankfurt and noticed another American who kept talking to himself about "where are the Frankfurt trains?" When the next one to Frankfurt came, I told him to get on with me to go to Frankfurt Hbf. He declined because there was no "Frankfurt" sign on the train. Oh well. . .you can lead a horse to water, but. . .