“Some use HOn3 track for On18 – I did. If you run the numbers, that 9 mm (0.354 in) gauge track actually comes out to a 17 in (792 mm). Since there was a real world 18 in gauge track we simply call this On18 .. rather than On17. 
On18 Car Width and heightIn order to ‘Fabricate’ an On18 clearance gauge we need to attempt to find the width of a car/engine. I found a thread on Trainorders.com where it goes:
That gave me enough information then to calculate the width of the car for On18 .. (PseudoEngineering again). If I keep the same proportion to the gauge that the Maine 2′ used .. then … 3.2 x 18 = 57.6″ or 4’9.6″ .. which I rounded up to 58″/4’10″. In O scale that is 1.2″ Fig 1 shows a 2ft gauge locomotive that I resized for 18″ gauge. Note that I came within a inch of the calculated width (yes .. dimensions are only ‘sorta kinda’ correct .. but so is this entire guesstamation I am conducting. 
All well and good. No – we humans don’t resize (other than expanding over time) so let’s keep the vertical dimension for the loco/car height from the 24″ loco of 85.4″ and apply that to our 18″ loco. This is where I am not sure of – we need to know the distance from the top of the rail to the bottom of the car to which we can add our 85.4″ (1.78″ O scale).
I see some use HOn3 trucks and so looking at the NMRA RP23 Bolsters page I see that the “Center Bolster Bearing Above Top of Rail” is 1/4″. Elsewhere I find that Floor Height for HOn30 is 7.2 mm (0.283″). So .. until told different I am going to go with … 0.283″ (13.6″) + 1.78″ (85.4″) = 2.063″ (99″) from top of rail to top of car, and 1.2″ for the car width.
Fig 2 then is a starting point with width, height to floor and height to roof (Version 2). On the right (Version 1) is the one I made up several years ago. I just noticed while updating this that .. yes .. I had the gauge wrong. *sigh* Thoughts ..

Dimensions P & EDimension E is 0.283″ – or – 0.317 – or – something else. Dimension P I *derived* from an On18 truck I am working on which measures 18.7 mm across the journals .. so I rounded up to 20 mm and split the diff for a 10 mm / 0.394″ P dimension. If we agreed on the current dimensions this could be called something like Gauge A – minimum dimensions .
If we waned to add a slanted/clerestory roof it would really add little weight and would make sense for humans to occupy. I did this with V1 of the gauge. 
On18 Gauge Version II (HOn30 trucks)For now I am calling this good to go unless feedback convinces me otherwise. Note that this is made for those using HOn30 trucks. I am making another version to use my own On18 truck. Next step is to print one and share the STL file. 
On18 Gauge Version II (On18 trucks)This is designed to use my own On18 trucks with 20″ wheels 
I am on the same page. I have been loosely using 20 and 24 in gauge as a guide. I deduced that the car width of roughly 3X gauge (was also backed by Mr. Fairlie) and came up with 5 scale feet for width and a similar 2 foot car height. The cab floor of the 24 in gauge Decauville 3 ton 040 also has a 24 in height.
Who would have ‘thunk it’ those years in math class but I have a lot of fun with these calculations. I think it may be simply that it’s like a detective in a mystery story .. we are peering into the .. .not unknown .. but certainly murky small narrow gauge past. It can’t hurt to have a fellow molders also part of the investigation!