As an avid model railroader, I haven't seen any model trains offered for sale at Walt Disney World -- however my memory of the Germany pavilion at Epcot is not entirely clear if they do or do not have some model trains for sale there.
That said, Walt Disney was an avid model railroader -- 'tis why we get to ride the trains at Walt Disney World and
Disneyland. He built a 1/8 scale railroad in his backyard, the Carolwood Pacific and named the locomotive the Lilly Belle after his wife Lillian.
Bachmann Trains has reproduced the Carolwood Pacific with the Lilly Belle locomotive in HO scale -- it can be seen at
their website and ordered through any reputable local hobby shop.
Some of Walt Disney's own model trains can be seen at Wilderness Lodge and I recall seeing some at Old Key West, as well.