Luxembourg to Koblenz

This is perfectly possible and a very scenic trip. You will have to get a bus or taxi from Luxembourg Airport to the main Luxembourg railway station (the buses are frequent but not very fast, and urban buses rather than anything more comfortable), and then get a train, changing at Trier. (You need an "RE" train at Trier, the "RB" trains stop at every station and are much slower.) You can find train schedules at www.bahn.de, but you will be using regional trains so there will be no price advantage in buying tickets in advance. Buying your ticket at Luxembourg station will avoid any worrys about getting to the station in time for a prebooked train. The journey is around 2 and a half hours from Luxembourg railway station to Koblenz, and Koblenz to Boppard is only another fifteen minutes or so by train not counting the wait for the connection.

The second half of the journey from Trier to Luxembourg is extremely scenic, as the train runs along the banks of the Mosel with castles visible at the top of the valley on either side.

Comments in other threads about posters like me doing something other than directly answering the question posted by the OP have not gone unheard by me. However,

I have to assume that this trip involves a discount airline. But if you're going through all those cities on the same day, why not fly to Frankfurt, a huge hub, and train more directly to Boppard? There are a million nice sights between Luxembourg and Koblenz, but it sounds like you will only see them from the cab between transit stations, and through half-closed eyes on the train. I liked Boppard fine, but I don't think it justifies this grueling trip. (All I mean by that last sentence is that there a million towns in Germany as, or more attractive than Boppard. Just because it's in Rick's book doesn't make it a must-sleep!) BTW, did you check that the KD boat High Season timetable starts April 18, 2014?