I once measured the overhang and under hang of the longest equipment I have (18" passenger cars, RK Big Boy, scale PRR T-1) and determined that 5.5" was sufficient on O-72 and O-54. 1/4" clearance.
My club's 3-track modules are 4.25" between centers but we avoid running long scale equipment on the middle track.
Lionel's Fastrack curves are 6" between centers. A safe distance on curves.