The distance between the inside rails is about 42". The total width is about 54" so the rails are about 6" wide. The walkboards very from ride to ride; I've been on wood coasters where I could easily touch the handrails...let me see, my wingspan extends about 27" from my armpit, and I'm guessing the running board on the train is about 6" wide. If we assume that the edge of the track lines up with the outboard edge of the running board, then the handrail would be about 21" from the edge of the track, with the walkboard in between.
I'd say the walkboard is typically 24" at minimum. On Son of Beast, I swear on some of those banked turns the walkboards are 4' or wider...
--Dave Althoff, Jr.