Frankfurt to Trier and then Koblenz to Frankfurt by train

To Trier: 2 trains: Train route is FRA to Koblenz, change trains there for Trier.

Tickets:

• The one-ticket option is the Quer durchs Land ticket (day pass valid on any regional train sequence at any hour) at 52/2 persons.

• To save a few Euros you can buy RMV tickets to Mainz (about €10/2) and use a Rheinland-Pfalz ticket (€30/2) for Mainz - Trier.

You don't get off the train in Mainz. Just stay on to Koblenz.

Schedules: click on "only local transport" at the db site to find trains valid for the above tickets.

Koblenz - FRA: for the train sequence you mention is €52 on the QdL ticket (valid for any regional train sequence that day.) But for most people 10:35 is WAY late for an international flight. I'd leave a couple hours earlier.

Buy all these tickets in Germany. There's no advantage to pre-purchase.

Yes the train station in the airport is easy to find. You don't need to plan this detail from here and get tickets ahead of time. There will be English-speaking agents there who can sell you the tickets and explain where and what. Easy.

We found that taking the Flix bus was cheaper, faster and easier than taking the train. It goes directly from the airport to Trier. The bus stop is at Terminal 1, in the bus parking lot.

Yeah, you need to get to the airport sooner than 09:00.

"Someone else suggested stopping to see a medieval castle on the way to Trier and I'm wondering if travelers think that's a good plan."

The train trip to Koblenz is Germany's most scenic train ride. You'll travel along the west bank of the Rhine River past many castles that you can spot from your window seat. Click on the castle icons on this map:

http://www.loreley-info.com/eng/rhein-rhine/castles.php

The Koblenz - Trier train travels along the Mosel River much of the way and is also very scenic:

http://www.zughalt.de/wp-content/uploads/2011/10/DB11321.jpg
https://www.der-takt.de/typo3temp/_processed_/csm_NEU_T_Reichsburg_Cochem_3_087e83a8d1.jpg

If you don't take a castle tour somewhere, you're shortchanging your visit to the region. Should you do that on the way to Trier? Probably not. You'll probably be a bit jet-lagged and you have bags to deal with (and most of these towns do not have station lockers.) But you have other opportunities...

1.) You can tour Reichsburg Castle in Cochem pretty easily during your bike tour, which surely passes through Cochem. Find tour information at this link. Reichsburg has an interesting tour but can't be called medieval; after it was turned to rubble by the French in the 17th century, it was rebuilt in a more modern fashion.

2.) Marksburg Castle is just a few miles south of Koblenz in the small town of Braubach - and the most authentic medieval castle in the area. You will pass it (on the opposite side of the river) on your train ride to Koblenz. But it sounds like you will have some time in Koblenz at the end of your trip, and Braubach is just 10 minutes south of Koblenz by direct train, so it would be easier to tour Marksburg at the end, I think. (You can leave your bags in Koblenz at your hotel or in a train station locker.)

Why not hit both castles if you can make the time?