17/32 Inch Scale

From LBSC, The Modelmaker, Volume 7, Number 4, April 1930:

I had better explain that as I have had more to do with 2-1/2 inch gauge locomotives than all the rest of the sizes put together, I will talk chiefly about this favorite size; but want to point out that it is NOT "half-inch-scale" but much larger. A half-inch-scale locomotive should run on a track of 2-3/8 inch gauge only. A 2-1/2 inch gauge engine is roughly 17/32 inch or 13 mm to the foot, and stands nearly 5/8 inch higher than a 1/2 inch scale engine, and is of course longer and wider in proportion. Therefore, brother loco-men, don't waste valuable space by cramping a 2-1/2 inch gauge engine into 1/2 inch scale load gauge. Such a job viewed from the front makes you think of a bulldog standing with is legs straddled out.

1/2 vs 17/32

by Bill Donovan of Real Trains Inc:

When the hobby of model railroading was first being developed it was more common to list a track gauge than a scale. The emphasis on scale came later with scales sometimes being "adjusted" to fit the available track gauges, other times a mismatch was simply accepted.

The National Model Railroad Association (NMRA) lists scales from Z to 1 inch but does not list a 1/2 inch scale.

There is an association in the UK called the Gauge '3' Society that models in 1/2 inch = 1 foot scale. From their website they state:

Gauge ‘3’ (or N° 3, as 2½ inch gauge was originally known) was one of five standard model gauges recommended by a subcommittee of the Society of Model Engineers on February 1st, 1899

They go on to explain that their association is not for trains to ride on and instead refer to the "National 2½ inch Gauge Association" which is also in the UK.

Sometime around 1900 a set of track gauge standards was formulated. Thus tracks with a dimension of 2.500 inches between the inner rail edges was designated "GAUGE 3". At that time, this gauge was fairly popular for garden or scenic model railways, with the engines using clockwork or meths powered. Certainly none of them were capable of hauling the driver, let alone a driver and passengers! Such capabilities arose from the work of (arguably) one man, Lilian (Curly) Lawrence, who wrote under the pen name of LBSC. Initially, the scale used for standard gauge locomotives was half inch, but this was changed to 17/32 inch (about 13mm) very early on.

However, in the US, Martin Lewis seems to be the source that many live steamers think of when discussing 1/2 inch scale. He offered kits for both a 4-6-4 and 4-8-4. His locomotives were smaller versions of what we build today. They were oil burners, had super-heaters, water pumps, fully equalized leaf spring suspensions and operated at 100psi. His plans and instructions state that the scale is 1/2 inch = 1 foot. Checking his wheel dimensions they come out correctly at this scale. They are for 2-1/2 inch guage track.

17/32 inch scale may be an attempt to better match the previously available track products that were already on the market at 2-1/2 inch guage. This is similar to the case of 1.6 inch scale today. The problem with this or any scale that is not a simple fraction is that if you need a piece of metal to represent say a 3 inch steel bar on the prototype, in 17/32 inch scale it comes out 0.133 inch which is hard to buy. In 1/2 inch scale it is 0.125 inch which is 1/8 inch and simple to get. For the home builder this is an issue.

Since both scales use the same gauge the track standards are the same. With the track being the same the wheel dimensions would also be the same, except for the wheel width. If a minimum wheel width of 6 prototype inches is used the width would be 0.250 inch for 1/2 inch scale and 0.266 inch for 17/32 inch scale.