Burg Eltz

Normally, a day trip from Bacharach to Burg Eltz would be possible. Of course, that depends on you starting fairly early and not spending too much time. If you are using a Rheinland-Pfalz-Ticket, €24 for two, you could leave at 9:21 and be at Moselkern at 11:10. You'd have all day to do the round trip. Return trips from Moselkern to Bacharach leave Moselkern until 9:41 PM.

Castles plural? Consider also the travel time and distance from Bacharach to Marksburg (Braubach), Rheinfels (St. Goar), and Reichsburg (Cochem, excellent falconry show) if you plan to tour more than one. It might make sense to stay in a castle town instead. St. Goar has maybe the most attractive setting, with 3 castles visible from town, and a ferry to the train lines on the opposite side of the river as well. Or stay in lovely Braubach, and you're only 12 minutes from the train hub Koblenz, where you must change to travel to Eltz. Bacharach, further south than any of these other towns, is 40 minutes from Koblenz. Staying further north also allows you to use the VRM daypass at 20 Euros for 2-5 people. A 3-day version costs 40 Euros. Bacharach is too far south to be included on this pass. Accommodations in the Rhine towns: http://www.bensbauernhof.com/accommodationsrhinemosel.html

Nadine, It is absolutely possible to visit Burg Eltz on a day trip from Bacharach, as I did that same trip last year. Due to a previous leg injury (and to save time) I chose to take a Taxi from Moselkern rather than the hike. There doesn't appear to be a Taxi service in Moselkern, so it had to get one from Löf. Thankfully a woman working at a small bar / Ice Cream shop at the end of town spoke English and was able to not only call a Taxi, but also translate my request for the Taxi to return to Burg Eltz at a pre-arranged time to take me back to the station. It was an interesting ride as the Driver spoke NO ENGLISH at all, and although a bit expensive it was the best method under the circumstances. The rail trip from Bacharach to Moselkern is ~1H:43M, with one change in Koblenz. Standard fare is €13.90 PP person each way (I don't know if discounted fares are possible on that route, but I'm sure Lee has that information). With the hike to Burg Eltz, the trail ends in the valley below the Castle, on a small meandering river. It's a relatively easy climb up to the main entrance. Check the Guidebook regarding times for English-language tours. Note that no photos are allowed during the "official" part of the tour, however you will be able to take photos in the Treasury after the tour. Although reconstruction work was underway at the time and scaffolding was in place, it's an awesome Castle and I really enjoyed my visit! Happy travels!

As pointed out in the other replies this is a very easy to do trip from Bacharach. You do want to watch that out of Koblenz you board a regional (RB - Regionalbahn) rather than a regional express (RE) train. The RB trains stop at every station while the RE trains stop only at larger towns. Also check the schedule for when the regional trains stop at Moselkern. A couple a of years ago we just missed getting back to Moselkern in time for the RB train and got to watch several express trains flash past - there really wasn't much to do in Moselkern. The express trains along the Mosel also make Trier a day trip from Bacharach.
We found the LED headlamps that sporting stores sell to be great for exploring the dark passages and tunnels at Rheinfels. Photos of what we saw based in Bacharach in May 2009 are at http://www.eclecticjourneys.com

I was at Burg Eltz last week and there was still construction work going on with scaffolding blocking the views and making for not very lovely photos. As the castle was closed I was unable to find out when they expect the renovations to be finished and could find no information on the internet about it either. It would be a shame to hike so far without being able to see it in its full glory, at least if you could know in advance then you wouldn't be disappointed.

The last sentence here states that work is expected to be finished in Fall 2011.

You will find the train schedules here. Bacharach is just OUTSIDE the VRM, Verkehrsverbund Rhein-Mosel, which covers an area from Oberwesel down the Rhein to the border with Nordrhein-Westfalen, just below Remagen, and up the Mosel to Bullay, so you can't easily use the VRM Minigruppenkarte (all day pass for VRM, €20 for 2). St. Goar is within the Verbund, so if you stayed there, you could use it. However, Bacharach is still within Rheinland-Pfalz, so you could use an RL-P ticket for not much more. The RL-P ticket is unusual for a Länder ticket in that the base fare, €21, is for the first person, each addition person is €3 more up to €33 for 5 total.