Any suggestion on rail trip Frankfurt to Rome.?
We have previously done Paris to Nice on TGV and that was good.

Frankfurt - Rome is a long bugger.
Very long.
The shortest travel time is a little over 12 hours with 3 changes of train, in Basel, Arth Goldau (in Switzerland), and Milan Central.

I think I would look for one of those cut rate air fares. Someone will know which one to use.

German wings has a flight from Köln/Bonn airport to Rome for €89 during the week and much cheaper on Sunday.

You can get from Frankfurt to Köln in about 2 hours with a few changes of train and the S-bahn.

The flight is about 2 hours.

I think your would be trading 6 or 7 hours against 12 or more.

The rail fare Frankfurt to Rome is about €200.

I think flying is your best bet unless you enjoy sitting on trains, which I don't, not even in first class.

You can check www.whichbudget.com or www.skyscanner.net for budget airlines that fly between the two cities. With Frankfurt, be aware that Frankfurt am Main is the airport closest to town and most readily serviceable by public transport.

Germanwings flies both from Köln/Bonn and from Stuttgart to Rome. Both airports can be reached from Frankfurt within less than 2 hours by train. Btw if you are landing in Frankfurt, DON'T go to the main station - the airport station has more and faster train services both to Köln and to Stuttgart.