Hey!

The most scenic routes are IMO covered by EURAIL pass and I’d recommend you to buy an EURAIL pass. The other one does only pay off if you also plan to use buses but most of the interesting bus lines so or so don’t run in December as the pass routes are having their winter break.

Scenic route: from Colmar go to Zürich. Then you’ve the opportunity to go by train to Chur, that line is already quite scenic for a main line (sit on the left side in driving direction!) - offering views of Zürichsee and Walensee with the famous Churfirsten.

From Chur you could spend a day exploring the Rhaetian railway (or get off in Landquart already - 10mins before Chur). The most interesting line is the Albula and Bernina railway which takes you first to Samedan/St. Moritz and then across the Bernina to Tirano in Italy. This is by far the best railway experience that’s possible on an EURAIL pass (of course also a bit depending on the weather). From Tirano you can catch a regional train to Milano (sit on the right side for views of Lake Como) and then from Milano you’ve frequent trains to Venice (reservation needed). 

It would be also possible to put one more scenic route in your itinerary: from Basel head to Interlaken, then spend a night there (with maybe trip to Jungfrauregion - not included in EURAIL). Next morning take the train from Interlaken to Lucerne and then from there to Zurich. Then do the same as written above.

Me and a friend have done a similar route working our way in the opposite direction from venice to Zürich. The only scenic route we did was the bernina line (highly recommend by the way). This takes you from Chur (an hour from zurich) to st. Moritz and then Tirano (in Italy). From tirano you can get to , and venice is only a high speed train away from there. 

The interrail pass does include the bernina express, though you will need to pay to reserve seats in the panorama coaches. We actually used the regional trains instead, they run on the exact same line and you are free to get off and take in the views a bit longer before continuing the journey. In summer (dont know about winter) they came hourly and most of the tourists were on the bernina express, so for us it was a lot quieter and more enjoyable.

Hope this helps.. if you are looking for more to do on the way to venice ive put up a picture of our route for inspiration... some of the gaps (i.e. by lake como have been boat trips). 

Advice for italy would be

1) be aware that many cities charge tourist tax when you arrive, for us in venice it was 4 euros per person per night. Make sure you have the cash to pay for it if your accommodation has not covered it in the price

2) all regional trains in Italy are covered in the padd as well as high speed trains. For high speed trains you will need to pay to reserve your seats.

For venice, i dont know how busy it gets in winter but for us in peak season it was very busy. Getting up and out early to be first at attractions before they open is a good idea. Dont pay ridiculous amounts of money for the gondola (80 euros for half hour per person). Buy a day ticket on the vaparetto (public transport boats) and you can not only explore venice but the outer islands as well. Cost us 25 euros each for the day and we went everywhere we could on them. If you buy them at the right time (they are 24 hour) you could use them to get back to the airport or train station.

Best get your seat reservations as soon as possible for every train with mandatory reservation. Especially Eurostar, although France can be nuisance as well during the Christmas holidays as almost the entirety of France will head to family to celebrate, so trains will in strong demand.

Get your Eurostar seat reservation here (or use it to check availability): https://www.b-europe.com/EN/Booking/Pass#TravelWish

For France, you cannot buy tickets yet for the Christmas holidays. Ticket sales open at 4th of octobre this year.

https://www.sncf-connect.com/aide/l-ouverture-des-ventes