Converting and Modifying an HO Scale Locomotive to HOn3

Combining a brass body based on a White Pass & Yukon prototype with the drivetrain and trucks of a Kato NW2, the author made a unique and reliable diesel to operate on his HOn3 model railroad.

Converting and Modifying an HO Scale Locomotive to HOn3

2021 HOn3 Annualby Rich Mossholder/photos by the author

I recently started the journey towards achieving my Master Model Railroader certificate from the NMRA. Not fully understanding the sheer level of work (and paperwork), I am slowly making my way forward. While reviewing the various categories and speaking with friends who have achieved their MMR’s, one of the common threads of discussion is, “Don’t make things harder than they need to be.” So with that in mind, I found a simple way to convert a standard gauge locomotive to operate on narrow gauge track.

For this article, I will explain the process I used for building the first of the three locomotives required for the Motive Power category. I wanted to build something that was different, using inexpensive materials, and be a little more whimsical. I am not one who is prone to a strictly prototypical adherence. I prefer to model what I like and I don’t care if running a Union Pacific Big Boy in New England seems absurd. I am inspired by modelers, who with tongue placed firmly in cheek, modeled their miniature world as they saw it. Folks like John Allen, John Olsen, and Malcolm Furlow were, for me, inspirational and fascinating story tellers in writing, photography, and especially, their modelling.


I needed a locomotive that was small but mighty to pull the MOW track cleaning train around the layout. I have an old blue box Athearn F7 which was modified to run on HOn3, but is too wide for my tunnels and bridges. I needed something narrower, but just as powerful.

I didn’t want to use my steam locomotives for this because they require more maintenance and don’t have the tractive force to pull the cleaning train without double heading. There aren’t many narrow gauge diesels available in HOn3 that will work for this, so let’s build one.

As I stated earlier, it must be inexpensive, reliable, and powerful. Old Athearn blue box locos are everywhere. They’re cheap and relatively easy to convert to narrow gauge. I made the initial conversion using an Athearn SW1200, which fit well and required no modification to the shell. I didn’t like the noisy gearbox, so I looked for an alternative loco-motive that I could convert. My thoughts instantly went to Kato for their smooth running characteristics and reliability.

Kato NW2

I have four HOn3 brass diesel shells that I purchased a couple of years ago from a friend. They are White Pass & Yukon General Electric “shovelnose” locomotives. I had thought about scratchbuilding the chassis and drivetrain to build them as prototype, but that is beyond my level of ability for the moment, and some parts would be challenging to source and near impossible for me to build.

While digging through some of my old boxes under the layout, I found a Kato NW2 that I bought when I was involved with a local HO scale modular club. To my surprise, the NW2 wheelbase was a great match for the shell. To top it off, the axles appeared to be very easy to convert to HOn3, much like the old Athearn blue box locos. The major difference between the NW2 and shovelnose were the number of axles. The shovel-nose has six and the NW2, only four. This was very concerning at first, because I know the rivet counters would be extremely critical of such an outrageous discrepancy. But then I remembered rule number one. “My layout, my rules!”

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This article was posted on: December 15, 2021