You'll of course want to take your sons on the WDW Railroad that circles the perimeter of the Magic Kingdom. There is a
behind-the-scenes tour of the Railroad, but unfortunately guests must be 10 years old to experience this. There is also a train ride at the Animal Kingdom that takes guests from the main part of the park to Conservation Station. This train is interesting because the seats all face the same way running the length of the cars rather than the width. You'll also want to point out the features of the monorail that make it similar and different from a standard train.
There are a few other places you'll want to explore. Outside the Germany pavilion at Epcot there is a very elaborate model train set up. This replica of a German village is very detailed and realistic.
The Iron Spike Room at the Wilderness Lodge Villas can be visited by any guest. This lounge-type room on the ground floor of the villas is dedicated to Disney train memorabilia. The room is not large, but it is very cozy and there are lovely photos of Walt with his trains and a beautiful train-themed firescreen.
For your own education, there is an excellent book called "Walt Disney's Railroad Story: The Small Scale Fascination that Led to a Full Scale Kingdom" by Michael Broggie. This book is out of print and is quite expensive used (check on Amazon), but you may be able to find it at a library. This would give you lots of stories to tell your sons about Walt and his trains. If you can't locate the book, an alternative would be for you to go to the website allearsnet.com and search for an article called "The Magic Behind Our Steam Trains."
I hope you and your railroad enthusiasts have wonderful trip to WDW!