Train travel from Frankfurt to Trier

I think the best and easiest way to get from FRA to Trier is to take regional trains from FRA to Mainz to Bingen to Koblenz (change trains) to Trier. I say best, because it goes through the scenic Rhein gorge between Bingen and Koblenz. For this route, you don't need advance purchase tickets; regional tickets and passes are the same price if purchased in advance or at the time.

You can find schedules and some fares at the Bahn schedule website. Put in Frankfurt (M) Flughafen Regionalbf
to Trier.

I looks like the all regional route (above) will also be the fastest. You will need a local RMV (Frankfurt metro) ticket from the airport regional bahnhof to Mainz Hbf for €4,95 pP and a Rheinland-Pfalz-Ticket, € 30 for 2 people, about €40 total. The RMV ticket is an open ticket, valid any time, but it probably won't have the date printed on it. If not, you will have to cancel (stamp the date) in one of the machines in the station (watch what others are doing). The Rheinland-Pfalz-Ticket will probably come with the date on it, and will be valid for unlimited rides on regional trains in the state of Rheinland-Pfalz on that day only. Mainz is a station on the border of Rheinland-Pfalz, and Trier is also in Rheinland-Pfalz, and the route I gave you, from Mainz, lies entirely in Rheinland-Pfalz.

You can take an S-Bahn from FRA to Mainz Hbf or a regional train to Mainz or beyond, but either way, even if you don't change trains in Mainz, you will change tickets (from the RMV ticket to the RL-P-Ticket) in Mainz.

Thank you, we were hoping for the most scenic route.

cpetz1027, assuming you are flying into Frankfurt airport? There is a train station (actually two) there in the airport. You can walk to it through the terminal. There will be a ticket office with English-speaking agent that can sort you out, or ticket machines that can show you the options and sell you your tickets. Easy.

Regional trains leave FRA from the Regionalbf, which is close in, under the building across the access road from Terminal 1. You get to it by going down stairs or escalators just inside the outside wall of the departure hall of Terminal 1 and under the road through a mall to the station. Sometime you might come from the concourse at the level below the departure hall, in which case you will go directly to the Regionalbf. Just look for and follow signs. Don't go to the "Fernbahnhof".

If you are coming in on Lufthansa or a Star-Alliance airline, you will come into Terminal 1. What airline are you flying?

You will be getting very detailed info about 1 month before your tour.

We are flying Lufthansa into Frankfurt. Thanks everyone for all this info. Looking forward to our trip!

Honestly, the easiest way to get to Trier from the Frankfurt airport is with the Flix bus. Goes directly there in 4-4.5 hours. Cheaper than the train, (23-27 € total for 2p.) no worries about changing trains and it is faster. You get this bus and the ticket from the bus area at the very end of terminal 1, at P-36. You will see signs directing you there. There are only 2-3 buses per day depending on day of the week.
https://shop.global.flixbus.com/
You might want to think about simply staying overnight in Frankfurt and going to Trier the next morning. After a long flight, do you really want to sit in a bus or a train for another 4+ hours? This way, you get a good nights sleep, have a great meal, walk around the city a little bit, you can take advantage of a cheaper fare, and you get to see another historic, German city!

Took this bus last year and was surprised at how easy it was.

I would not recommend the bus because a) it's a bus, b) the route will not be as scenic, and c) it does not run as frequently as the train, and is more likely to be delayed in traffic

We took this bus last year in July from Frankfurt to Trier and there were no staus, it had great AC, and it was really scenic. Actually, better scenery than many train rides I have done in this area.

Actually, better scenery than many train rides I have done in this area.

Looking at the shortest routes highway per ViaMichelin for FRA to Trier, none of them go via the Rhein gorge, which should be the objective of the most scenic route. Per ViaMichelin, a hwy route south of the gorge is an hour faster than the route via the gorge, so I really expect that Flix bus takes the fastest route, not a route via the Rhein gorge.

Although I'm not averse to taking buses in Germany, in this case I can see no justification for using the bus to save a few euro (€27 vs €40 by train) and missing the most scenic route (and adding an hour). And the one change train connection only takes 3 hours.