I've modeled a number of roller coasters with different CAD programs. The hardest part of trying to recreate a ride in 3D or even creating an original yet realistic design is coming up with the dimensions. I have often wondered what the rail center to center distance is. I've searched high and low on the internet but it is not something that is easily found. Well, today I finally got some definitive data. The center to center rail distance on the Big Thunder Mountain roller coaster at the Magic Kingdom in Walt Disney World is exactly 30 inches. Each rail has a diameter of 3 inches.