HO Scale track spacing

[TMo]

If it's straight track sections, my rule of thumb is 2 inches center to center, but if you're placing rolling stock between two trains it's nice to have a bit more room for fingers. On curves, I'd up that to 3 inches, but in those cases, it'll depend on the length of your rolling stock. Prototypical spacing is around 14 or 15 feet I believe with some variability.