I really liked the California Railroad Museum Sacramento. They have a good railway bookshop. I was recently at the Sugarpine Railroad near Yosemite, about 4 hours drive from SF. So a mere hop by American standards.
British Airways flies directly from London Heathrow to Denver. A good option, as changing planes in America is stressful and unpleasant.
On may way from Denver to Durango, I stopped at the St.Elmo ghost town. No railway sights, but it's the wild west as you imagine it!
There are lots of good tips abut the Cumbres & Toltec and Durango & Silverton railraods in this topic
https://www.gscalecentral.net/g-scale-travel/cumbres-toltec-or-durango-silverton/
My pictures from that trip (you can see them without logging in to facebook)
https://www.facebook.com/media/set/?set=a.10151068819299044.457562.506409043&type=1&l=86b445f940