As a railfan I think your plan is great. The Cumbres and Toltec Scenic RR from Antonito, CO to Chama, NM is also bucket list material.
You may want to post an inquiry on the Roads and routes Forum about the best way to get to Durango. Somewhere, you will have to tow over the Continetal Divide. If you take I-40 to Amarillo, TX you can take the US87 cutoff NW into New Mexico and pickup I-25 at Raton. Raton Pass is easy at just over 7000 feet. Then it's I-25 N. to Walsenburg, CO. There you would take US160 West all the way to Durango. On US160 you will go over Wolf Creek Pass at 10800 feet. We have only driven a car over that pass so others could advise you about towing on that route.
Glad you are coming to see us in Colorado!