The S-Bahn is a very good commuter line that connects various cities in the Frankfurt area:
https://en.wikipedia.org/wiki/Rhine-Main_S-Bahn. The contempt for DB that Rahsaan speaks of is for longer trips, and I've never had a problem with the S-Bahn.
Among other things, you can get to Hochheim where Künstler is making fabulous wines. Domdechant Werner is also there, and if you take a taxi, Joachim Flick is only a few km away and making wines well worth knowing.
There is a train from Wiesbaden that goes along many of the Rheingau towns; I've never taken it but I think it will give you access to many top producers within walking distance from the train stations, including P.J. Kühn, Wegeler, Spreitzer, all of whom are making great wines. Unfortunately, I think you probably can't get to Robert Weil, Kloster Eberbach (especially for the historic cloister, which is no longer where the wine is made), Schloss Johannisberg, Prinz, or Johannishof without a car or taxi, but if you have one, they are definitely worth the visit.
Lot of producers in Rüdesheim, but above all, Georg Breuer, which has extensive visiting facilities:
https://georg-breuer.com/index.php?s=e_vinothek. Leitz is there, as well, although I'm not so wild about recent vintages. It's a big tourist town, so there surely are multiple locations where you can go to taste many producers at a time.
The Städel Museum in Frankfurt is less impressive than you would think for a city of Frankfurt's size and importance, but it is worth a few hours' time.
Mainz is the center of the Rheinhessen, but off the top of my head, I can't think of any producers located there. The Romanesque Cathedral is definitely worth the visit. There are local trains that would take you down to Nackenheim, Nierstein, Oppenheim, Dienheim, etc. where you can visit producers such as Gunderloch, Brüder Dr. Becker. Wittmann, Keller, Groebe, Winter, Wechsler, Dreissigacker are inland, so I'm not sure how easy it is to visit there via train, although IIRC, Keller is not far from a train station.
Check out the websites of the tourist agencies of the various towns, villages, etc., they will have plenty of information.
Sorry, no restaurant recommendations that I can think of right now.