Rhine by train in March '26

I would suggest Boppard, since you get a free VRM transit pass for the length of your stay.

activities to do in the region (that will be open)

• Marksburg Castle in Braubach (includes a 25-minute uphill hike from town... or reserve a taxi up.)

• Rheinfels Castle in St Goar (if you actually end up staying in the Middle Rhine Valley... St Goar or Boppard.. but I don't recommend that in mid-March. See suggestion below for just visiting Marksburg)

• Mid-march is too early for river cruising, chairlift rides, and outdoor stuff.

Suggested approach for 4 days:

I suggest you pay a visit to Cologne first. Spend one night there and catch some of the sights. Germany's most-visited landmark is Cologne Cathedral. There's a WW II memorial site to victims of the Holocaust at the NS Doc Center. And several museums as well.

I'd move on to Mainz the next day for the remainder of your time. But on this travel day, stow bags at Koblenz station (on the way) in a locker on a stopover of several hours and take the train to Braubach (which is only 10 minutes from Koblenz by train) for Marksburg - and take a walk through this very attractive old-world town with half-timbered buildings too. Back to Koblenz for bags, then off to Mainz. You'll probably reach Mainz in the afternoon sometime. Suggest you stay in the old-town zone near the Dom (cathedral) and Marktplatz.

That leaves 2 days to sightsee in Mainz. There's quite a bit there. And it's a fine place for wining/dining and picking up gifts before flying home from FRA. There are direct trains from Mainz to FRA airport starting in the wee wee hours of the morning - trip takes 25 minutes.

https://www.tripadvisor.com/Attractions-g187393-Activities-oa0-Mainz_Rhineland_Palatinate.html

I'd pass on the whole idea. Nothing will be open in those cute little towns along the Rhine in mid-March. It will be cold and grey, no leaves on the trees, no river tours running, and there's a possibility of snow. Between Rosenmontag (16 Feb, 2026) and Easter (5 April) everything is shut. And frankly, in the area between Mainz and Koblenz even Fasching isn't a big deal. From New Years to Easter is the worst time to visit that area.

Now staying in Mainz or Koblenz might be worthwhile, depending on what you want to see or do. There are good choices in both locations. Frankfurt also is worth several days depending on your proclivities.

The Rhein will be really dead in March. One place that isn't though, is Eltville. Eberbach Monastery is a fun bus ride away and this place should be at the top of any trip to the Rhein area. The basilica here is gorgeous! The movie, Name of the Rose, with Sean Connery was filmed here.

The trains on this side of the Rhein will easily take you to Frankfurt, Rüdesheim, Wiesbaden, Koblenz or the Marksburg.
Tons of stuff to see and do in Frankfurt of course (dozens of museums and lots of historical sites, a 1000 years of Jewish culture, as well as medieval churches, great farmers markets, and the Kleinmarkthalle).
Rüdesheim may still be shut down for the season, not really opening up until Easter.

As others have said, the normal summertime stay in the most scenic part of the river (south of Koblenz, north of Bingen/Rüdesheim) is a weak idea in the middle of March - my main reason for suggesting Cologne + Mainz instead of Boppard.

Eltville has been mentioned - an attractive town you might want to spend some time in. The location of Eltville is convenient if you end up staying in Mainz - Eltville is quite close by, and like Mainz, it lies upstream a ways, outside the most scenic part, on the opposite riverbank from Mainz... iI's almost a Mainz suburb and can be reached with just a 30-minute train ride. See this map of the Rheingau vineyards.

As for seeing Marksburg in Braubach: While it's possible to reach the town of Braubach by train from Eltville, Wiesbaden, or Mainz, it becomes an unnecessarily long train trip, like 2 hours round-trip, one that makes you backtrack north in the direction of Koblenz and Cologne. This is the reason I suggested hopping off in Koblenz for a Marksburg-visit stopover (a very short trip to Braubach) on your way south from Cologne to Mainz.

Along with Marksburg Castle, the half-timbered buildings of the Rhine Gorge will be on show in Braubach, by the way, no matter what season it is. See photos of Braubach on the 3 pages at the link below:

https://www.stadtbild-deutschland.org/forum/index.php?thread/7322-braubach-am-rhein-und-marksburg-galerie/

I suggested Eltville as a place to stay, not just visit. It has tons of half-timbered buildings, a beautiful church, a castle tower, wineries, hotels, businesses are open, lots of great restaurants and the scenery is gorgeous. You have easy access to all the towns I mentioned as day trips.
It only takes an hour by train to get to Braubach from Eltville. Coming back, you can stop at Lorch, St. Goarshausen, Rüdesheim or Assmanshausen. No, none of these places are mentioned in the RS book, but that doesn't mean they aren't worth seeing. Just be aware that they are really, really empty and dead in March.

No, none of these places are mentioned in the RS book, but that
doesn't mean they aren't worth seeing.

True. The RS book and Rick's other materials tend to emphasize the Rhine cruise and the town of Bacharach over everything else in the Rhine Valley. Many nice places do not get a mention at all, as Jo indicates... and it's not just those few towns on the east-bank railway which she lists that get overlooked. Braubach and Osterspai (I made tiny Osterspai my east-bank base town for 5 nights on one occasion) are outstanding little places as well. East-bank railway map with towns:

https://rheingaulinie.de/files/rheingaulinie/files/Fahrplan/fahrplan.png

West-Bank Railway towns that his materials also overlook or minimize include Boppard and Oberwesel.

To be fair, Rick does mention Rüdesheim... AND Mainz... using one sentence for both in my older edition... "They're not worth your time." Now, that's just wrong, IMHO, and my sense is that there are many other forum members who have been there who think the same.

I don't disagree with Rick at all that Bacharach is worthwhile. The half-timbered buildings there are very nice. But the town is deadsville at that time of year, and there's little-to-nothing to do indoors there anyway. It's also a difficult place to just stop off at if you have luggage with you (no station lockers.) If you do a side-trip from Koblenz to Braubach + Marksburg as suggested, jenny, you'll have the Rhine's top castle tour as well as a nice collection of half-timbered buildings. Whether you stay in Mainz or in Eltville or somewhere else south of the Rhine Gorge, I'd definitely do this side-trip to Braubach.

If you end up staying in Mainz as I suggested, you will have time for a day trip to Eltville, possibly including Rüdesheim as well. If you end up staying in Eltville, you will have time to visit Rüdesheim as well, and perhaps to make a day trip to Mainz too. Either way is possible. My gut says that Mainz would be the more practical choice for a couple reasons...

1) March weather will encourage you to be indoors, and you will have more to see and do indoors in Mainz.

2) Mainz is better situated if your final destination is Frankfurt airport. From Eltville it's an hour plus with a change of train, but from Mainz it's just 25 minutes and direct. Mainz is a good place to spend one's last night before flying out. (If your destination is Frankfurt itself, then Mainz is closer too, though not much - only about 10 minutes closer.)