What train would I use to travel from Seville to Frankfurt? Is this even feasible or am I better flying from Seville to Frankfurt? If so, any and all suggestions are most welcome. I thought train travel would be more relaxing and give me opportunity of seeing the countryside.
It might be slightly faster than 25 hours now with the TGV service (a few years ago it literally was about 25 hours). But yeah, fly it in 2 hours. I LOVE trains and love the scenery and relaxing, but anything over 5-6 hours is a flight now. And it'll be far cheaper.
If you want to investigate what's involved by train, use the Bahn (German Rail) website, following Rick's tutorial: http://www.ricksteves.com/plan/tips/db_tips.htm.
Unless you want to break up the trip, I'd recommend flying. If you do want to break up the trip, you can use the "via" option on the DB website to get alternate routes.
You're on the right track with the recommendations to fly.