Given one’s geographic limitations, you may find obtaining a copy of “Tourist Trains Guidebook – 4th Edition” published by Kalmbach Books very helpful for selecting something of interest within reach.
Not to slight other areas and offerings, but Colorado south and west of Denver may be the US gold standard of tourist excursion railroads. Having very recently been to Colorado, I strongly concur with Don’s suggestion of the Cumbres and Toltec RR. The recently restored and reconfigured parlor cars are exceedingly well done and recommended. A vintage narrow gauge steam excursion at its finest.
If you are in the general area, one must also strongly recommend the iconic Durango and Silverton. Currently, the track is in better shape on the C and T yielding a smoother ride, but the D and S has the even more spectacular scenery and offers a very interesting tour of the shop and museum. The significant other will likely find Durango more appealing than Alamosa.
While we are discussing Colorado rail sights, do take the Royal Gorge Route Railroad out of Canon City. The Royal Gorge offers smooth diesel service with available full dome cars with dining service and truly spectacular views of the route of the California Zephyr in its D & RGW glory days.
If you can also arrange to do so, the Pike’s Peak Cog Railway with its Swiss-built equipment is a must. Beware that is considerably cooler on top of the peak than in Manitou Springs. The views are breathtaking (literally) and some can briefly con themselves into believing that there really are prototypical 3-rail railroads.
Steamtown in Scranton is always very worthwhile with its live steam train rides, museum, and working turntable and repair shop.
While you are in Scranton, be sure to visit the Electric City Trolley Museum and take a ride on one of the Brills such as the beautifully restored Red Arrow Lines example shown below.
Enough for maybe two summers, hey?
Bob