I have visited many working steam railroads around the US.
But, not all of them...
My favorite for an overall experience is the Cumbres & Toltic Scenic Railroad.
I prefer to ride the train out of Chama, NM due to the train has to climb up to Cumbres Pass at 10,000 feet in elevation.
Ride all the way to Antonito, CO so you can experience the two tunnels. There are bus options available.
There is an open car that all passengers can visit. There is no seating available in this open car, standing only. The country land is very open and you can get many, many images of the locomotive working hard and going around curves. Lots of hard chugging sounds and plenty of steam/black smoke blowing!
An extra day would be required to "chase" the train after it leaves Chama. You can capture many, many images of the whole train as it makes its climb up to Cumbres Pass.
Also, the yard is open to the public. You can wonder around the yard all you want. Just don't go into the shop as there are open pits and the management does not want you climbing on any structures or equipment.https://cumbrestoltec.com/
Check youtube for more videos.
My favorite to learn about how a locomotive works and to ride behind one is to take the Disney World, "Behind the scenes" tour in Florida.
This is an early morning tour before the Disney Park opens up. Catch the train at 7am, I think (and don't be late or you will miss the tour).
This tour is unique in that you get a very good explanation of how the locomotive works from outside the locomotive and a tour/explaination of the controls inside the cab. https://www.youtube.com/watch?v=qX9QOyxl_IQ
The video is about 36 minutes long.
The above Disney tour is in Florida and is very similar to the one I attended.
Just a note, If you watch the above video, you can hear the music (that is played at the beginning of the tour) even there are no visitors inside the gates. Kinda weird....
So, that's it.
If you visit, please post images!!!