Using specially manufactured 16.5 mm (0.65 in) gauge track, scaled at 7 mm to the foot (with appropriately spaced, larger Railroad Tie underneath – United States O scale rolling stock (1:48), it becomes a narrow-gauge track of 2 ft 6 in (762 mm), and is referred to as “On 2½” (or On30, as in 30 inches).1Wikipedia – O scaleThis is based on HO track. If you run the numbers, that 16.5 mm (0.65 in) gauge track actually comes out to a 31.18 in (792 mm). Since there was a real world 30 in gauge track we simply call this On30 .. rather than On31.

NMRA S-7 Clearances

This is from the NMRA S-7 Clearances It states that “NMRA Type II Gauges used the Old Time dimensions for Narrow Gauges.“. With that in mind, here I have pulled just the O Scale Old Time Dimensions and Gauge.


  • A – This is from the centerline to the outside of the body so represents half the width. This 1-1/2″ is 3″ wide for the body which full-size is 144″/12′ or Old Time Dimensions or Narrow Gauge – meaning 3 ft gauge.
  • For On3 this is A = gauge x 2 (36″ x 2 = 72″). Car width would be twice this or gauge x 4 = 144″
  • For On30 this would be A = gauge x 2.4 (30″ x 2.4 = 72″). Car width would be twice this or gauge x 4.8 or 144″. If we keep the same multiplier we used for 3 ft narrow gauge (gauge x 4) then we would come up with  A = 30″ x 2 = 60″. Car width would be twice this or 30″ x 4 = 120″. Therefore, A = 1.25 in / 31.75 mm
  • Since there is a direct correlation between car width and gauge and stability I will modify the Old Time Dimensions for On30.
  • I took 6mm out of the center which made P and G 35mm. I kept the 19mm for B and changed the angle down to D.

Will this work with Bachmann On30? Have not idea – they may need to use the On3 gauge – which I will also create.

On30 Gauge

The last thing to consider is the locator tab that fits between the rails.

  • Width : 16.5 mm (0.65 in) gauge track – since I need the tab to fit ‘easily’ I am reducing the width by .1mm to 16.4 mm (0.646 in) – for now at least. I may need to modify that after printing a gauge.
  • Depth: Looking at the Flange Clearance chart on Brunel Hobbies’ Flex Track & Rail Facts page we see that a 0.035″ depth will clear the spike on Code 55 rail.
    • I cranked off this gauge .. call it version 1.