While the majority of narrow gauge railroads in the U.S. were 3' gauge, there were actually quite a few that were 2-1/2' gauge. Mallory Hope Ferrell ran a series in the Gazette recently where he chronicled some of them. Most were logging lines or really short lines with a few to several miles of track.
In the model world, the number after the "n" represents the gauge in either feet or inches. On30 and On2-1/2 are therefore the same. Bachmann introduced their On30 line so that O scale trains could run on HO scale track. As Doug said, this works out to 30" gauge, while On3 is 36" gauge.
In the model world, the number after the "n" represents the gauge in either feet or inches. On30 and On2-1/2 are therefore the same. Bachmann introduced their On30 line so that O scale trains could run on HO scale track. As Doug said, this works out to 30" gauge, while On3 is 36" gauge.