"I hadn't considered it (the train to Trier.) How is the scenery?"
Not bad on the whole, completely over the top once you pass the Frankfurt/Mainz area and arrive in Bingen.
Scene at Bingen
Downstream from Bingen
A bit further yet downstream
Your group meets at 4 pm Saturday in Trier, takes a pm walk there, and spends Sunday touring the city, right? Sadly, the Best of GAS tour does not provide any ground time in the scenic and historic Middle Rhine Valley, where these photos came from. But you can fix Rick's mistake. On Saturday morning, why not catch a train to Bingen, drop your bags, have lunch, then take the 14:30 Rhine Cruise to St. Goar? It takes just 1.5 hours and once there you'll have time to tour Rheinfels Castle if you want - or just take a walk and enjoy the scenery.
Bingen has a developed riverfront walk dotted with cafes and restaurants, great for strolling. So catch a train back to Bingen after you've finished with St. Goar - the train covers the same gorgeous territory but with a different perspective.
Book a river-side room at the NH Bingen at the downstream end of the waterfront if possible. It's a great location on either side. Guests staying on the town side of the hotel can enjoy the river scene from the breakfast dining room as well.
I see a Saturday 10:13 train out of Bingen that gets you to Trier by 12:30. Earlier and later options are also possible. From Koblenz the train follows the Mosel River most of the way - expect ooh/aah scenery here too.
At the DB site I found a Friday May 20 7:46 departure from Munich to Luxembourg and added a 22-hr. stopover in Bingen. The fare was €36.90 each. This "Super-Sparpreis Europa" fare allows an extra-long stopover, whereas a standard ticket to Trier would not. Refundable "Europa" fares are also available for more €s.