The age requirements for tours are very strict. You would not be able to bring a six year old on the
behind-the-scenes tour of the Railroad. However, there are many other train-related activities. You'll of course want to take your sons on the WDW Railroad that circles the perimeter of the Magic Kingdom. 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. 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 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." You may also be interested in the Carolwood Historical Society (website at carolwood.org). They are dedicated preserving information about Walt Disney's love of trains.
I hope your railroad enthusiast has wonderful trip to WDW!