My boys love trains, especially my 14-year old, so it was an easy choice for us to check out the Railroad Museum of Pennsylvania. Since we have a large family, it was the same price for us to become a member as to buy tickets for the whole family for the day ($40). With the membership, you get free admission to the museum for the year. I am suspecting that we will be back again!
As well as the hundreds of locomotives on display, some of which you can go inside, there are lots of hands-on activities for kids to do. There is a simulator in which you can pretend to actually drive a train. The screen responds to your motions, and shows what you would see.
There is an area in which kids can pretend to shovel coal (black foam shaped like coal) into the firebox.
A display of the system used to sort mail on the train allows kids to sort plastic cards into the slots just like they did on real trains.