train from Frankfurt airport to Trier Hbf

"I'm trying to find the most economical price for 4 people (2 adults, 2 teens). "

The QDL works but you can pay less with a 2-ticket strategy...
It's €16.40 for 4 on a group day ticket (Gruppentageskarte) bought from an RMV ticket machine at FRA. This gets you as far as Mainz.
It's €39 for a Rheinland-Pfalz ticket for 4 adults (assuming here that your teens are 15+) for Mainz-Trier. If one teen is a child (free,) subtract €5.
So w/ these 2 tickets, it's €55.40 total vs.€68 (or €60 if one is under 15) for the QDL ticket, same routes, same regional trains. There is no price advantage for pre-purchase. Buy tickets at FRA's Regionalbahnhof station from a ticket machine with an RMV logo.)
The only restriction here is that your train must leave Mainz after 9 am. If that's not the case, just buy the QDL. However... the QDL won't let you leave FRA prior to 9:00 am (whereas the RMV ticket I suggested DOES allow that.)

The above prices are for regional trains. To find eligible regional trains, just click on "only local transport" when you do your search.

Otherwise, for InterCity and InterCityExpress trains, the Flexpreis is the rate you'd pay at the station. Savings fares require advance purchase and lock you into a particular departure

The high-speed (IC, ICE etc.) connections take a variety of routes to Trier - they might route you via Koblenz, Cologne, or Mannheim; the regional train itineraries via Saarbrücken, oddly enough, are the faster way to reach Trier (about 3 hours for the RE/RE options) because the route is more direct than the high-speed routes. So... look for those RE/RE connections with one change of train for the fastest and cheapest options.

Thank you all - So I guess the best bet if I don't want to be locked in to a particular train/time is just to wait until I arrive in Frankfurt airport and buy the tickets then, allowing for any flight delays.

There are english-speaking staff at the train station there that can help.

Adding a reply so I can find this later! We're doing this tour in September- looking for information on the train to Trier. Seems relatively straightforward!

Anyone have any further tips?

Get the DB app for your iPhone/iPad...it is WONDERFUL. We used the app to great effect last year and had no problems figuring out train schedules. I just ran a quick search from Frankfurt Airport to Trier, on a weekday, there are 15 train options if you board between 8 am & 12 noon. Also, the DB train offices in the train stations are a great help too. Many have English speaking attendants who will help you. Don't forget to reserve seats too, otherwise, you may get on an overcrowded train with no place to sit. Guaranteeing seats may cost your group of 4 about $20EU extra, but it is worth it.

"Anyone have any further tips?"

• I want to mention that the ticketing advice I provided noelle was specific to her circumstances and may not be right for all parties.

• IMO taking a train ride right after a long overnight flight is OK, but a 3-4-hour train ride to Trier would be TOO long. I would much prefer to arrive a day earlier at FRA so I can have a hot shower, get my inner clock adjusted, settle in and get a night's rest - then meet up with my tour group the following day. (Mainz is a good place for this and for some sightseeing - it's on the way to Trier but just 25 minutes and €5 each by train from FRA.) Then the next day, take the train to Trier whenever it suits me. The 11:51 RE train sequence from Mainz gets into Trier at 14:30, just about right for a 4 pm meeting (Rheinland-Pfalz ticket, €24/1 or €29/2 persons.)

noelle wrote, "So I guess the best bet if I don't want to be locked in to a particular train/time is just to wait until I arrive in Frankfurt airport and buy the tickets then, allowing for any flight delays."

• Yes - buy at the airport station. BUT... If the train sequence you take uses any of the long-distance / high-speed IC, ICE, or EC train options, you will pay a lot for the train ride to Trier - between €80 and €90 if you are a couple. (Add €9/couple if you want reservations, but be aware that you'll only have reservations on the high-speed trains, not on any of the regional trains you might need to take to reach Trier.) The R-P ticket, the €29/couple fare I refer to, is valid only on the regional trains. To find these trains and these trains only at the DB website, you must check the "only local transport" box when you do your search. If you fail to check this box, you may pull up SOME of the regional train options but not all - and you will pull up itineraries that are not possible with the R-P Ticket.

I assume you can do the same with the DB app mentioned by the previous poster.

• Reservations are generally not possible and not necessary on the regional trains.