what is the center to center measurement for HO for parralle tracks
thanks
ray
There’s no precise answer to this.
The best I can offer is that 2.5-3" is common, and would allow most locos/rolling stock to pass each other.
Much less distance is often seen in yards, stations, branches, etc.
Even more is done for high-volume mainlines, or those with lots of long cars (e.g. full-length 80’ passenger cars can come into play here)
And where would I suggest the largest center-center parallel distance of all? Staging tracks/yards. Leave plenty of room to ‘fiddle’ the trains with your hands in the one place that is very likely to happen a lot.
ditto
Technically, concentric curves are parallel at each radius that passes through them. My point, though, is that it is on curves that you must give your future use of the layout some room to pass. Overhangs on long cars and engines can cause hang-ups if the curves are too close.