There are some fine railroad museums in the United States. They have a large collection of historic rolling stock, from replicas of the earliest steam locomotives, to modern diesels. The displays are well done, and the staff is helpful and knowledgeable. The California State Railroad Museum is another good one. They have historic equipment, and displays that will help you to understand how the railroad work.
Several pieces are in dioramas, including a snowshed and narrow gauge train suspended on a trestle.
They feature many historic locomotives from the Pennsylvania Railroad. Best of all, they're right across the street from the Strasburg Railroad, so you can also ride a steam train while you're there. Tourist Railroads Railroad museums are wonderful, and they do a good job of teaching people about trains and what they were used for. However, seeing a steam locomotive in museum is like seeing a plane in a hangar or a boat in dry-dock. You really don't get the entire picture of what it's all about until you've ridden one.