It's been quite some time since I've been to
Disneyland, but I believe the train tunnels/dioramas were created out of the need to "hide" the real world from guests. In this Bing Maps image, you can see the two diorama buildings between Tomorrowland, Harbor Blvd, and the what's now a bus loop.
Second and more importantly, I don't think there is room. The Disneyland trains have guests riding sideways and only seeing into the park. The Disney World trains have guests facing forward. That means they'd have to have something to see on both sides of the train. The buildings above would need to be maybe twice as large.