I was like that 5 year old, and I still am now! The locomotive named the "Flying Scotsman" has superstar status certainly in the UK, maybe elsewhere, so much so that the really dyed-in-wool railway enthusiasts know her by her number, 4472. (Locomotives, like ships, are female, even with a name like 4472's, and you only had to see her at work to know that she is a lady.) I'm getting choked up here. When she left Bristol Temple Meads on a special steam tour about 10 years ago, I saw grown men wiping away tears.
I would have thought that if a person is mad about railways, the NRM would be sheer paradise in earth, and it would be invidious to select one exhibit over another. They cover the whole range of railway history over the last 200 years. And it's free. Spend a day there. Go by train. Railsaver UK have a web page with a "spend the day in York" section with some suggestions for places to eat, things to do, etc.
http://www.railsaver.co.uk/nationalexpress.htm"Meet at Café Puccinos, up in the station’s old signal box, for coffee and a bit of trainspotting. Then head across the footbridge for the short cut to the National Railway Museum. You can easily spend a whole day here, and it’s free. My son’s runaway favourite is the Japanese bullet train carriage, but you can also see the enormous turntable in action, step inside 1930s sleeper carriages and see engines being mended behind the scenes. There’s a good outdoor play area, too. Those with a head for heights can take a spin on the new Yorkshire Wheel next door. And kids love the “road train” that takes you into town. Good places for a picnic lunch include the nearby Museum Gardens. Then head to the picturesque Rowntree Park, on the banks of the River Ouse, with its play areas. Make time to wander through the city and see the numerous street performers (bound to cost you a few quid). And you may be tempted to blow the budget on wooden toys at Christmas Angels. For tea, your best bets are Pizza Express, five minutes from the station, and the famous Bettys, wonderfully high-quality yet surprisingly child-friendly with a good kids’ menu. Finally, take a walk back to the station along the city walls—a free experience that’s hard to beat."