The National Railway Museum is one of the best free museums in Britain, and not only for people who care about trains. The Great Hall is a converted steam locomotive depot, and its sheer scale — combined with the variety of engines inside, from early Victorian to modern Japanese — produces a genuinely impressive effect. Royal saloon carriages used by Queen Victoria and later monarchs are in immaculate condition. The working turntable allows locomotives to be repositioned, and on good days you can watch them move. Allow at least three hours; the museum is bigger and better than you expect.
The collection is split between the Great Hall, the Station Hall (with carriages and smaller exhibits), and a workshop area where restorations are in progress. The Mallard, the steam locomotive that holds the world speed record for steam traction (202.58 km/h, set in 1938), is here. So is a replica of Stephenson's Rocket and a cross-section of a Channel Tunnel locomotive. Children do very well here, but the museum is by no means aimed only at them. Entry is free; there are charges for some special events.