Frankfurt - Rhine Cruise - Burg Eltz quickest possible

Burg Eltz is not on the Rhine. It's near the Mosel. The closest train station is Moselkern. You can either get there by hiking or taxi. I don't think it's something I would attempt after a long flight as a stop on my way to Amsterdam. I don't think it makes sense to consider it unless you do spend at least one night in the area.

The key is in the statement, "possibly stay overnight in Koblenz". If you do this, the first day of the trip is certainly doable. Assuming the flight does get into FRA pretty early, take a train to Bingen Stadtbahnhof or to Bacharach, cruise to St. Goar, then continue by train to Koblenz.

The second day, if you are willing to get to Amsterdam around 7 PM, you must leave Koblenz around 2 PM, which means leaving Moselkern around 1:30 or Burg Eltz by 12:30, depending on how fast you hike. You should be able to see Burg Eltz and leave by 12:30 if you leave Koblenz early enough in the morning. Note: there is also a bus to and from Burg Eltz from Hatzenport, but only on weekends in the summers.

Trying to do it in one day - never.

Are you aware that Burg Eltz is not the only visitable castle in Germany? Some of them are on the Rhine. For example:
https://www.ricksteves.com/watch-read-listen/read/articles/three-rhineland-castles

https://www.ricksteves.com/watch-read-listen/read/articles/rhine-castles-a-turret-studded-meander-along-the-rhine

This is a personal opinion, but I thought the modern interiors of Burg Eltz were the least interesting part of the visit. If you're in a hurry, make some compromises.

cruise the Rhine while the sun is good for pictures

What times do you consider "good for pictures"? Most photographers would call sunrise or sunset "good for (landscape) pictures". Is that going to work with your tight schedule?

Tim has a good point. I've never been to Burg Eltz (On one opportunity it was raining and I didn't want to hike in the rain. The next time it was already closed for the season.), but I can't imagine a castle that has been continually lived in would look like it did hundreds of years ago. On the other hand, the Marksburg, in Braubach, the only other intact castle in the area and closer to Koblenz than Burg Eltz, was given to the German Castle Association hundreds of years ago, and they have attempted, at least, to keep it as it was when it was a working castle. The Marksburg would be a very viable alternative to Burg Eltz and might take less time.

Great responses.

Lee, good advice all around.

Tim, I know there are other castles, but we're on Rick Steve's Forum and if he says Burg Eltz is the place to go, I'm going there.

Lubitsch, it's not a hit and run. When I was in Germany in 2014 I planned on taking a Rhine cruise after visiting Fussen, Munich and Rottenburg, but because of time constraints I had to cut it out so it is a continuation of something I wanted to do. This trip I am planning is mostly Amsterdam, SE Britain and Edinburgh and I only get a 2 week vacation so sacrifices have to be made. Also, there's a direct flight form MCO to Frankfurt so why not start with those 2 things on the bucket list.

Anna, I wouldn't even call myself an amateur photographer. I guess what I was trying to say is I want to hit the Rhine in the afternoon when the sun is high and I can take decent pictures on both sides of the river of the castles high up on the hills without sun spots on my photos.

You're too modest... Why be satisfied with "decent" pictures if you can get great ones? I challenge you to at least try getting some pictures of the Rhine valley at sunset or sunrise! :-)

"When the sun is high" is the time for photographers to take a nap. ;-)

BTW, Rick's comment, as linked by Tim, that the Marksburg tour is only in German, is incorrect, or at least outdated. The castle's website, says that Englisch [sic] tours are offered at 1 and 4 pm. I'm not sure I was there for that time, but I did have an English tour in 2004. I remember because there were four of us on the tour, a young Czech man who lived in Canada and spoke English, his Canadian girlfriend, who only spoke English, his aunt, who lived in Germany and didn't understand English, but took the English tour with her nephew, and me.

We took a german tour of Marksburg. We were given an English cheat sheet, but the guide also very kindly gave us a quick summary at every stop.

xray,

When are you planning to take this trip? Note that the Castle will be closed until 24 March 2018.

The castle is some distance from rail lines, and you can either hike up from Moselkern or use one of the options listed on their website - http://burg-eltz.de/en/ (look under Planning Your Trip).

When you arrive at the upper car park, you can either hike down the somewhat steep hill to the castle, or take the shuttle van (costs about €1-2 each way).