What's the difference between O Scale, On30 Scale, and On3 Scale?

On30 Trains

Size matters, right? Truth is, they're all O scale. The difference is track gauge. Track gauge is the spacing between the rails - from inside rail to inside rail. O gauge, On30 gauge, and On3 gauge, are all O Scale (1/4 inch equals one foot).

  • O gauge is 1.25 inches between the rails (inside rail head to inside rail head). O gauge represents prototype standard American railway gauge modeled in O scale, that ran on prototype track spaced at 5 feet between the rails.

  • On30 or originally On2.5 foot gauge is 0.65 inches between the rails (the same gauge as HO scale track). On30 represents prototype narrow gauge modeled in O scale, that ran on prototype track spaced at 2 feet 6 inches between the rails.

  • On3 represents prototype narrow gauge modeled in O scale, that ran on prototype track spaced at 3 feet between the rails. On3 gauge is 0.75 inches between the rails.

Simple, right? You see the problem is when you set down real prototype railroad ties and then nail in rails, you can put parallel rails as close or as far apart as you like. It depends on the distance between the wheel flanges on the rolling stock and locomotives you want to run on your railroad. There wasn't a standard gauge in this country for a long time. It didn't matter at first, because you ran your own stuff on your own railroad. Once railroads started sharing equipment and service routes, it became an issue. It gets even more complicated if you want to use European gauged equipment and track.

Just remember, Scale, is the modeling dimension - 1/4 inch equals one prototype foot in O Scale. Gauge is the width of the rails. O, On30, and On3 gauge are all O Scale models that run on different gauge track.


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  • FineTrains on

    Hi Nelson. You’re wondering if the modern prototype track gauge of 60 inches is close enough to Sir George Stephenson’s original 56.5 inch standard track gauge from the early 1800s, in O Scale? To be clear, we are talking about a prototype track gauge difference of 3.5 inches or 0.07292 inches in O scale. That’s a hair over 1/16TH of an inch. If that bothers you, then there is no other choice than for you to hand lay all your own track and modify all the wheel spacing on your models. Why settle for less?

  • Nelson Lawry on

    I have long been interested in railroads and steam locomotives, and have been published in Trains and other major RR mags. Am now grappling with model railroading with thoughts of getting my own small layout (I did write SMALL). Your explanation of the three common O scale gauges is quite clear, including the compromise of using readily available HO scale track for a 2 1/2-foot gauge RR, against the more common North American 3-foot narrow gauge RRs. My question: Because O gauge track arithmetically represents 5-foot or wide gauge, what O gauge track is precisely equivalent to 4 ft 8 1/2 in standard or Stephenson gauge? Or at 1 1/4 inches between the rails, is O gauge simply “close enuff” except for perfectionists who would hand-lay their own track and modify the wheels and trucks on their gear? Just a bit perplexed here.


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