I can tell you a bit more about the underground system at WDW without getting in trouble, so here goes.
There IS an underground system, however only at 1 1/2 of the parks. At the Magic Kingdom, you are actually on the 2nd floor, 14 feet in the air. This tunnel underneath, called the utilidor, is how you get from one area of a park to another without ruining the theming (IE, would you want someone who's dressed to work at the Haunted Mansion cutting through Adventureland? Nope.) At Epcot, there is a tunnel underneath Future World, but not underneath World Showcase. You just walk behind the buildings at WS to get from one place to another, but many of the attractions in Future World are accessible by guests from all 4 sides, hence the need for the Tunnel.
In the Disney MGM Studios, there is no tunnel. Every attraction has a backstage exit either in the attraction or adjacent to it so you don't have to walk far to get to a non-guest area. However, since it can be a long walk from one end of the park to another, and the tunnels would have really sped up your trip, we are provided with Schwinn bicycles to get from one place to another backstage more quickly. Say a Manager has to get from the Tower of Terror to Muppetvision. Those are on opposite sides of the park, so they'll hop on a bike and significantly decrease the time of their trip. Most every attraction has at least breakrooms inside/below where the guests go. There's one right between the 2 drop shafts in the Tower of Terror, and the walls move in when an elevator drops by (get it? Drops by?)
That's all I should really say, there's some stuff I'd like to expand on, but I don't exactly know what Disney will do, so I'd rather not risk it. This isn't really inside information or anything, and what I've just said is all common knowledge, but I'm a new guy and don't want to get fired, so I'll shut up now.
Jeremy