Lake Como to NE France by train

I suggest taking the train 1 hour down to Milan and then fly up to perhaps Brussels. Then work your way west and south. It all depends on where you want to go in France and the stops along the way.

Lake Como is south of the really big mountains, and they're a physical barrier that's just a little to difficult to deal with. Most of the trains go toward the northwest out of Lugano.

then travel, scenically, concluding in NE France, by train

The obvious choice would be the train from Varenna to Tirano, then over th Bernina Pass to Pontresina, stopping for a few days.

Then to Chur and down the Rhine valley to St Margarethen, Bregenz to Lindau. Stay in Lindau or elsewhere along Lake Konstanz for a few days. Then train to Singen and the Black Forest Railway to Gengenbach, staying there for a few days. Then train to Offenburg, Kehl, and Strasbourg, staying there for a few days. Finally, TGV to Paris.

We took a Italy/France trip one year where we started in Venice & ended in Paris. Our route between Italy & France where you are probably interested was Torino, Annecy, Lyon. We enjoyed all three of those locations.

Many thanks for your helpful suggestions!