Although the long trips are confining and not up to the old days, there are a number of small trips that are wonderful.
1. Cumbres and Toltec. This takes the mountains on the border between New Mexico and Colorado. From Chama NM to the Cumbres Pass (10,000 feet) and Antonito, CO on the old narrow guage trains of the former Denver & Rio Grande Western. The best part is from Chama to the Passand on to a lunch stop, and return to Chama; takes a day. Cumbres
The site is under revision at the moment, but the map is available.

2. The Napa Valley Wine Train in California, Napa to St. Helena. Great food and wine, a one-day trip if you include the car trip from Sacramento or San Francisco to Napa. Napa Wine Train
One of the great rail experiences. A good selection of chef-prepared food and many wines available.
The rail museum at Sacramento is, I think, the best in the country.