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Found this idea on the net but was written up in Kalmbach's "Model Railroad Planning 2018". The issue featured Doug Tagsold's Colorado & Southern. The premise is using HO gauge equipment to represent 1/72 scale narrow gauge. Liking out-of-the-box thinking this is my take on the idea.

Doug had already dismantled a large On3 layout in his lifetime so this isn't his first narrow gauge rodeo. Here is a the reasoning from Doug on why 1/72 from the article:

What should this endeavor be called?
Everything should have a name. Thought of "Scale72" but "Scale1/72"  pronounced as "scale one seventy two", has a better ring and more closely defines the scale. Since there will probably be no standard gauge we probably don't need a gauge designation just "scale one seventy two". When I did 55n3 should have made it 1/55n3. That would have been clearer about the scale.

Click here to view Doug's video

Click here to view a BlogSpot about Doug's layout featuring videos

Were there any narrow gauge railroads close to four feet? The Arcata and Mad River was 3'-9-1/2". There were several others near four feet in the early days of narrow gauge.

Click here to download a scale ruler

You first need a scale ruler
For those who choose to pick the nit
Were there any four foot gauge railroads?

The Helca and Torch Lake was 4'-1" gauge and even had a Camelback loco.
The Bachmann New Tooling Oldtime HO 4-4-0 becomes an early narrow gauge 4-4-0. I had a printed cab made at Shapeways as a direct replacement for the HO cab.
Doug Tagsold used the MDC/Athearn/Roundhouse HO 2-8-0 for his locos. The loco mechanism matches the first generation narrow gauge consolidations. 14mm Scale Link drivers give us the correct driver size. A boiler from the Tyco/Mantua Ten-Wheeler gives us a boiler.
The Roundhouse does make a credible C-Class narrow gauge loco, that plan is a C-19. The drivers are a bit large but they can be changed out easily.
There are drivers and axles available from places like Scalelink in the UK.

These are 13mm which will give us Scale172 36" drivers. Scalelink has 3mm self quartering axles for the newer Roundhouse/Athearn locomotives along with 1/8" for the older original models.

The internet is great.

Freight Cars
The Bachmann OldTime HO boxcars scales well against this Ohio River and Western early box. Just needs to be lowered and better trucks added.
The Mantua/Model Power boxcar finally scales out to something prototypical. Just needs some little tweaks. Here is our work in progress Mantua conversion. From $11 dollar toy train boxcar to a 24' narrow gauge in an evening.
An IHC cattle car with Tahoe trucks.
Bachmann gon with new trucks and lowered.
 Bachmann bobber is about a foot narrow.
Here is a Roundhouse caboose during my "OO/HO in the 1870s" experiment on the old PcalRwy.
Passenger Cars
The Bachmann HO passenger car's windows match this early D&RG car but the Bachmann car is larger.
The Bachmann HO passenger fits the body dimensions of this OR&W typical second generation car.
Structures, people, wagons and animals
The OO world supplies us with many thing. OO figures are only a mm too short, an easy fix. OO early vehicles are close enough.
Preiser supplies 1/72 animals and figures but seem to be in short supply.
HO and OO structures lend themselves to Scale172.
Seems like Deja Vu all over again
Back in 2005 I started my "OO/HO in the 1870's" project. Eventually the project was abandoned when sound couldn't be put into the IHC HO 4-4-0s I was using.

The twenty percent difference between 1/87 and 1/72 is a lot. It is easier modeling in a larger scale.

Back then I was using shortened 1/72 figures to go along with the OO/1/76 figures. Here is my IHC/Roundhouse 4-4-0 merge. The little guy on the walkway is HO. That is a really big difference.

Sound back in 2007, eeeek!
Scenes from the old PcalRway in 1/76
Here are scenes for the "OO/HO" project, this is what 172n4 will look like. The Roundhose cars are good examples of later narrow gauge cars. Doug used the Roundhouse cars in his endeavor.
Easy Narrow Gauge in "Scale One Seventy Two"
MDC/Roundhouse/Athearn Cars
 Doug Tagsold's Colorado & Southern used Roundhouse/MDC cars which work great.
 Doug Tagsold's idea is great if the goal is to build a narrow gauge layout. Everything is there. Almost "Ready to Run" and cheap.
But does the gauge really matter if it is possible to build narrow gauge easily and inexpensively?
The Roundhouse Mogul has two versions the more common high wheeler and a scarcer low driver version which has a 1/72 43" drivers. They mechanically match early narrow gauge moguls .
The boiler is easily lowered and smaller drivers applied.
Bigger cabs are required to remove the HO look. The stock cab can be bashed to make a taller one. The opening can be fitted with a new filler piece and all that is required is an 1/8 high skirt around the bottom. A new side panel would be added for decoration. Our crude mockup with a cab from another project.

New S-scale C-16 domes are the right size to make our Scale172 C-19. Those domes are Tomalco but domes are available from Slim Gauge 3D at Shapeways. The motor mount just has to be adjusted and the cylinders have to be modified to lower the boiler.

Why not S Scale?
The question is "Why not S Scale?". Then we lose our C-19, our 1880s 4-4-0, our one evening 1870s box and no evening 1880's box. The HO equipment doesn't translate to S Scale. Can live with the gauge to have that equipment.
Been there done that in S Scale narrow gauge. S is really limited in equipment. The available HO equipment to rebuild to S is shaky. The Tyco ten-wheeler hasn't held up well over time. S scale is very expensive. Just using the equipment on HO track as Sn3.5 doesn't change anything.
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The Tichy HO ore car would make a great coal "jimmy". Can be had for $5 a piece on the street.
Things are sometimes better than what the box says. The second guy from the right is a OO scale Bachmann Branchline figure. The guys on his right and left are Preiser 1/72. The guy on the far left is a Langley Victorian OO.
What About Standard Gauge?
Sn3 cars scale out well as standard gauge cars. On3 or On30 trucks can be widened to Scale172 gauge of .784. OO scale 12mm diameter wheels are close enough. The Bachmann On30 Mogul could be massaged to represent a standard gauge loco.
Late Era  Narrow Gauge
The Roundhouse/Athearn HO 2-8-0 scales really well against post-1900 narrow gauge 2-8-0s like this Southern Pacific narrow gauge 2-8-0.  The stock cab can be adjusted to make a late 2-8-0 as here in our mockup.
The most important thing is whether you believe the concept not the naysayers and nit-pickers. The hobby is all about fun.

"Most visitors won't notice the difference of this scale. To them, the C&S is
simply an HO standard gauge layout with a narrow gauge feel."
- Doug Tagsold
Locomotives were proportional the same setup can be used for a HO early era consolidation.
Why not just do HO?
We now have our Scale172 trains running. Simple mods to HO equipment gives us narrow gauge in a larger scale in the 1870s. This was my reason for doing this. All the other scales can't give me easy 1870s narrow gauge. The Mantua boxcars need extensive modifications to make them 1870s HO standard gauge cars.
Adding 28" HO wheels gives us Scale172 24" wheels and completes the look of early narrow gauge.
The wheel on the ore car is a HO scale 33" and demonstrates the difference. A really D-oh! moment, wondered why I had an extra wheel set.
What About Track?
We are using HO Kato Unitrack to represent our early era narrow gauge track. The funky alternate universe HO spacing looks great for our track.
In our HOn30 adventure we found that the track can be painted at the bench and blended in with "fur grass" or Sculptamold/latex paint mixture.
A better perspective as a black and white.
While searching for a suitable proto railroad was perusing books from my late brother's collection. Found "Milwaukee Road Narrow Gauge" and found these pieces of narrow gauge and actual 1/72 scale drawings. The box matches the size of our Mantua with the same truck configuration and they had an 1880 caboose that is the same size as the Roundhouse caboose.
 Doug Tagsold  used the MDC cars with the HO trucks with 33" HO wheels. Substituting 24"(HO 28") wheels and shorter wheelbase Tahoe trucks makes the cars more "narrow gaugey".
Reviresco Cars and Trucks
Reviresco has a line of 1/72 vintage automobile kits.
Dart Castings supplies a line of OO scale wagons and figures which are well done. OO scale stuff scales out well in 1/72.
With minor modification HO oldtime equipment can be modified to achieve that "narrow gauge" look.  Figures and vehicles are available.