The DB train schedules are showing lots of irregularities on Germany's most scenic train route between May 2 and June 13 for Mainz - Koblenz and Koblenz - Cologne, where repairs will be underway. Buses are being scheduled to cover individual sections of track which become unusuable. Some scheduling changes have yet to be made, so what you see at the DB site for these dates right now is probably unreliable.
The notices below (in German) provide most of the details.
https://www.zuginfo.nrw/download/1743602901233_Allgemeine_Fahrgastinfo_RB26.pdf
https://www.zuginfo.nrw/?msg=114885
Between Mainz and Koblenz, the repairs and the schedule irregularites are on the "LEFT SIDE" Rhine railway, where the popular towns of Bingen, Bacharach, Oberwesel, St Goar (Rheinfels Castle) and Boppard are situated.
If you are coming from FRA or Frankfurt to the Middle Rhine area, as many do, it might be wise to use the "RIGHT SIDE" railway that follows the Rhine River from Frankfurt Hbf (the main station) to Koblenz via Wiesbaden and Rüdesheim. (From FRA, use the express bus or train to Frankfurt Hbf first.)
Right side railway: https://rheingaulinie.de/files/rheingaulinie/files/Fahrplan/fahrplan.png
If you are looking to book in towns with easier train access during this period, then Rüdesheim, Kaub, St Goarshausen, Braubach, or other right-side towns might be best. From the right side, you can still visit some of the towns on the left side; Rüdesheim, St Goarshausen, and Filsen are blessed with both train stations AND ferry crossings, which make Bingen, St Goar, and Boppard easy to reach:
- Filsen > Boppard ferry
- St Goarshausen > St Goar ferry
- Rüdesheim > Bingen passenger ferry (Bingen-Rüdesheimer)
And of course, the Rhine Cruise day boats by K-D and Bingen-Rüdesheimer also serve Rhine towns on both sides of the river.
Because the right-side railway also serves Koblenz, those heading to Burg Eltz or Mosel River destinations may want to use this railway instead of the impacted left side route. To find schedules from Frankfurt to places like Cochem, Trier, or "Burg Eltz, Wierschem" at the DB site via the right-side railway, just enter an intermediary right-side town (like Kaub) as a stopover with 00:00 stopover time.