We use 4 inch spacing almost everywhere. In yards...straight sections 3.5" works.
Many people ask this question, and it is next to impossible to answer. The correct answer is it depends on the sharpness of your curves and the length of your longest 2 pieces of equipment.
We use curves ranging from 72 to 96 diameter. In places where there are parallel curves: 96 and 89--pictured below. That leaves a shade under 4 inches between lines. No issues. Our longest 2 pieces of equipment: 21 inches. No sideswipes!
My suggestion is to physically test prior to actual securing the track in place.
