Although British railway museums set the bar very high, and definitely outdo the rest of the world for mainline excursions at speed with historic rolling stock, we do have a few respectable collections and displays in the US. Pennsylvania seems to be the mother lode.
The Railroad Museum of Pennsylvania, at Strasburg:
Across the road at the Strasburg Railroad, a high-quality operating facility in the heart of Amish country:
East Broad Top, an operating preserved 3-foot gauge coal company railroad, and Rockhill Trolley Museum, at Orbisonia:
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Steamtown, at Scranton, is a National Park Service property and has static displays, operating locomotives, and a working shop where heavy repairs and restorations in progress can be viewed by visitors. The Electric City Trolley Museum, with operating streetcars, is adjacent, and the Lackawanna Mine Museum, an intact but inactive anthracite coal mine, offers museum displays and a mine tour:
Washington, Pennsylvania, a few miles south of Pittsburgh, is home to the Pennsylvania Trolley Museum. There are operating cars, static displays, and restorations in progress:
There's lots more in Pennsylvania, and many other states and towns have museums ranging from busy operating facilities to rusty stuff enclosed in barbed-wire fences behind dusty, long-vacant depots.