I have read that many roads used to narrow their gauge to 4ft 8in to help steam engines and passenger trains run smoother at speed. It apparently reduced the sideways motion from the alternating cylinder strokes. Going the other way, roads with a lot of heavy freight traffic would sometimes widen the gauge to 4ft 9in, to reduce rolling resistance.
I believe, but wouldn't swear, that those were the limits for mainline track into the 60s, so half an inch either way. For industrial and yard tracks, the limit is sometimes the point where the trains derail, but passenger trains are definitely NOT permitted on such track.
