Hello, Jasmine!
What a great question!
When my son was just about that age, we were waiting to board the Walt Disney World Railroad after a fun-filled day in the Magic Kingdom, and my son just started sobbing. (His dad wasn't able to make this trip with us, and the fatigue of the day just had him missing Daddy all of a sudden.)
A Cast Member started chatting with him and directed us to step aside while allowing the other passengers to board the next train. At that point, we were escorted to very back of the train, where my son was given some stickers, was announced as today's "guest conductor" and was allowed to pull the handle that blows the train's horn! We rode at the very back of the train along with the real conductor, in a very special area until the park's entrance, with a smile on his face that lasted for days. It was one of those truly magical Disney moments, and it truly made our week!
I'm sure that, if you explain your son's love of trains, that you might just get a similar experience, too!
In addition, there's another really great train at Disney's Animal Kingdom, the Wildlife Express Train. I'm sure that your son will really love that train as well!
Check out the Wildlife Express Train Attraction page on Walt Disney World's website for more information.
Have a magical first trip! Be sure to ask a Cast Member for a "My First Visit" pin, too, so everyone will know that this is your little one's first experience with WDW magic first-hand!