Train Question in Germany

Getting off early is not a problem.

1. It's not like an airplane where they have a passenger list and check it between every stop. Nobody is going to know you got off early.
2. The ticket prices are "by the mile" (cost based) not like airline tickets that vary with the popularity of the stop (market based), where the ticket price might be more for an ealier stop because people want to get off at that place.

Years ago, there was a low cost airline flying out of Colorado Springs, non-stop, to eastern cities for very low fares. United matched the fares from Colorado Springs, but all of their Colorado Springs flights went through Denver, where the price to eastern cities was higher. So, it was cheaper to fly from, say, Detroit to Colorado Springs, through Denver, than it was to fly just Detroit to Denver.

Passengers soon figured this out, so they would book Detroit to Colorado Springs, for a low fare, but just get off in Denver. United, of course, had a passenger manifest for the Denver to Colorado Springs leg, and if you didn't fly that leg, they wanted to charge you the higher Detroit to Denver fare if you weren't on the Denver to Colorado Springs leg.

I'm not sure what eventually happened, but I think United realized that charging people extra not to fly them made them look kind of stupid.

I wasn’t sure whether it would be the same train that stops in central Frankfurt AND the airport. I guess that it is and we can just get off early.

The particular ICE that you list - ICE 26 - starts its trip in Vienna and goes all the way northwest beyond Cologne and ends in Dortmund.

The bit you are interested in is

Würzburg Hbf arr 16:22, dep 16:24, platform 7

next stop is Hanau Hbf arr 17:15, dep 17:16, platform 102

and then Frankfurt(Main)Hbf arrive 17:36, depart 17:42

and Frankfurt(M) Flughafen Fernbf arr 17:55, dep 17:57, platform Fern 7

All this is perfect for leaving the train at Frankfurt Hauptbahnhof at around 17:36. The Frankfurt main station (no platform noted yet) is a dead end station on that floor, just leave the train and walk towards the front. When you have passed the front you will be in the cavernous station - lots to eat - Frankfurt on Foot has a Youtube video of Ms. Jo walking around the station, looking at all the food. It was shot in January last year, the folks have masks on but other than that it is much the same https://youtu.be/8XjgorRPVHY.

That will be absolutely fine. You don't need to tell anybody you will be leaving a stop early, nobody will care, there is nothing to prevent you to. Just be sure to pick up all your luggage.

Happy travels - wish I were with you.

Note that the first two trains are regional trains, and you will not have reserved seats on them. Where did you book them? As the suggestion to look for assigned seats is confusing. Also be aware that if you miss a connection due to a late running train you can just take the next train without any formalities. Taking the train is like taking the Subway. Just with nicer trains and higher average speeds...

Thank you Nigel, Frank, Lee and WengenK, I feel assured now and not fretting that I have purchased a tkt we can't use.

Note, you have 8 minutes to make the change of trains (and platforms) in Würzburg. Plenty of time, but don't dally. Be at the door, with you luggage, when the train stops in Würzburg. Follow the herd to the stairs down to the tunnel. When you get to the tunnel, look for the sign for track (Gleis) 7. It should be about a hundred feet from the stairs you came down. Walking the hundred feet shouldn't even take half a minute. Go up the stairs to track 7. If you've done all this in less than 6 minutes, the ICE won't even be there yet.

If you have reserved seats, and if you have time, find the chart between the tracks that shows your train and where on the platform you need to stand for the coach with your seats.

It looks like you will be headed to the airport the next day. The fare will be 5,80€ per adult. There are both S-Bahns and regional trains going from the Hbf to the airport. The S-Bahn is a little faster, but you will probably have to go down to the underground station to get the S-Bahn. There will also be a few regional trains leaving from the ground level tracks at the Hbf. They take a few minutes longer, but they are easier to get to.

Or, you might want to walk a shorter distance (½ km) up to Hauptwache and take an S-Bahn from there.

Both the S-Bahn and the regional trains go to the close-in Regionalbahnhof under the transit building across the street from Terminal 1. You'll go under the street and come up stairs directly into the departure hall of the terminal. If you are flying Lufthansa or a Star Alliance airline you will leave from T1.