Are reservations for travel between Frankfurt and Bacharach really needed on weekdays? Can either go direct via Mainz or via Koblenz in first week of June. Deutsch Banh is advising reservations not possible. My wife and I are travelling on a Select Senior Euro Global Pass 30 days travel. is there a charge for a rail reservation when using a pass?
If reservations aren't possible, then it's probably a regional train and you can't make reservations no matter where you buy tickets. These regional trains can't sell out. Some trains can fill up even on weekdays; sometimes kids ride them home from school, etc. - maybe not completely full but two seats together may be harder to come by. Unless you have a lot of bags I personally wouldn't worry about it, but you can try to arrive early at the stop to make sure you get on the train first and be more likely to get two seats together.
You cannot "go direct" to Bacharach from Frankfurt (or Frankfurt airport) as a change of train is needed in Mainz, or possibly in Bingen, depending on the type of train you use. (Can't imagine changing in Koblenz unless you just want to schedule in a lengthy detour.) The trains you'll use will be local/regional trains, so reservations are not possible (they're only for high-speed trains, basically.) You can uses your railpass on these trains of course.
Thanks for the replies, kind of what I figured; I am used to trains filling up, worked in Japan for four years and used rail extensively there, haven't seen really full until you've rode a train in Tokyo, Yokohama or Osaka at rush hours. I was an airline pilot before and flew in and out of FRA a lot but we didn't go anywhere always flying out within 18 hours or so and of course ground transportation was never an issue since the carrier always arranged that to and from the airport and hotel.
As indicated above, the fastest route (about 1½ hr) from Frankfurt Hbf to Bacharach is by regional trains, which are not reservable. Specifically, only regional trains from Bingen (Mainz) and Koblenz stop in Bacharach, so you HAVE to arrive on a regional train.
If you really want to go at least part ways on a reservable connection, and don't care how much longer it takes, there are a number of direct Intercity (IC) trains per day from Frankfurt Hbf to Koblenz. Some take as little as 1½ hr, some longer. You can get seat reservations on these trains for 4,50€/seat. That's on top of the rail pass, which is usable for these trains.
In Koblenz you would take one of the hourly non-reservable MRBs to Bacharach. That leg take about 37 minutes.
You could also schedule an all IC connection from Frankfurt Hbf to Mainz to Bingen and an MRB (11-12 min) from there, but Mainz to Bingen only takes 16 minutes, so it's hardly worth the extra change.