I know these have been mentioned here before, but we just returned from a 4449 mile road trip and did a couple train things that I HIGHLY recommend:
1. Bailey yard & the Golden Spike Tower in North Platte, NE. The yard is over 8 miles long and 1-2 miles wide and handles about 150 trains a day. You can easily spend hours here (and we did) watching real trains - hump operations, all kinds of switching, through trains, refueling, replenishing sand, even a locomotive overpass. Binoculars are a must and a long telephoto lens is a plus. While we were there, a UP retiree that worked at Bailey was available to answer questions. I learned more about prototypical train operations in 2 hours than I have in the last 25 years combined!
2. Cody Park, also in North Platte, NE. A UP Challenger and a DD40x on display and with open cabs you can enter. The center opening between the 2 prime movers on the DD40x is large enough to walk through and we did. These 2 are HUGE!
3. The Black Hills Central RR. Located in the Mount Rushmore area, this is a tourist operation where we rode behind a Baldwin 2-6-6-2T, likely the only one in operation anywhere in the world. The route includes a 4% grade so with 8 or 9 loaded old coaches, we even had a few instances of the drivers slipping momentarily.
These three alone are worth a trip, but the (relative) proximity to Mt. Rushmore, the Crazy Horse Memorial, Devil's Tower, the Badlands, The Black Hills, Wind Cave, Ellsworth Air Force Base (B1B Flight operations, museum and Minuteman Missile Silo tour). Made us proud of the USA.