Travel from Stockholm to Frankfurt

Don't look on RailEurope, look for train fares and schedules on the Deutsche Bahn website.

Yes, forget about RailEurope. Always use the websites of the national rail operators for full schedule and fare information. Some of the cheapest rates I'm seeing for Stockholm to Frankfurt range from €89-99. BUT... the duration of the trip is anywhere from 18 to over 24 hours, and invovles multiple train changes. And that's only the statioin-to-station time, it doesn't include packing, hotel check-out, getting to the station and the reverse procedure on the opposite end. To me, the extra money spent on a flight would be well worth the lost vacation time you would spend on the train and then recovering from your travel day. Also, I can't comment on the rail-side scenery on the Swedish and Danish portions of the trip, but from Hamburg to Köln (one of the likely stretches you would take), the views are pretty unimpressive. Köln to Frankfurt is a little better, but the train moves so fast and through so many tunnles that you really can't see much anyway.

Yes, the most efficient use of rail would probably be the City Night Line train from Copenhagen to Frankfurt after 5 hour a direct Stockholm-Copenhagen train. Adding the comfort of a sleeping compartment will make the $200 flight look more competetive.

For all your flight options, look at http://www.skyscanner.com/. Look at both one-way and round trip fares; you can buy a one way ticket and just throw away the second part (don't throw away the first part, or the whole thing is voided).

On skyscanner also check Stockholm to Germany overall as you may find a good deal to a nearby airport. Also, I don't believe you need to bother checking round trip fares as the airlines don't seem to inflate 1-way fares (at least the discount carriers I used).

Brian: The discount carriers do indeed price all flights as one-way flights. But the "legacy" carriers like SAS and Lufthansa often charge much more for a one-way flight than a round trip flight. That's why I recommended checking both options.