On18 Clearance Gage

NMRA S-7 Gage
S-7_gageThis is the NMRA S-7 Gage that the first three rows of the table are referring to.
On18 Gauge
On18_gaugeThis is a ‘work in progress’ .. just fun. Don’t freak out.

Some of us have played around with using N gage mechanisms in O scale to create what we call On18. It actually measures 17 inches but that just sounds wrong so we go with the On18 tag. I was looking at some of my On18 track (actually Peco HOn30) where it runs along a backdrop and considering putting in a “Low Relief Flat” between the track and the backdrop when I realized I really had little idea how much room a loco would need – in other words .. I needed a On18 clearance gage.

I pulled up the NMRA Clearances Standards page and went to work.

In the table below, the O, On3/On30 and On2 rows are taken directly from the NMRA data. The On18 row is my own creation with the “Reasoning” below that. Feel free to chime in and help me work on this – in fact .. PLEASE comment, make suggestions and generally join in the discussion.

O Scale1-7/8"1"1-1/2"3"1"5/8"1-1/2"5-1/2"1-1/2"

I originally made the body wider – but measuring the clearance I have on my On18 line I *reasoned* to make the gage fit that measurement. If we toss this out and re-work the On18 passenger car so that a couple of people sit in a single row of seats to one side of a aisle, then the car width can be reduced quite a bit. If so then the width could be reduced to that over the trucks – G – 15/16″.


  • For On3/On30 this is the same at 1-1/2″
  • For On3 this is gage x 2
  • For On30 this is gage x 2.4
  • On2 is 1-7/16″ which would be gage x 2.875 which would give a body width of 11-1/2 ft.
  • On18 – I was looking at the interior of a Maine 2-foot passenger car. There were two rows of single seats seperated by an aisle. Since On18 is some six inches narrower than this I decided that the DRC&C would (if it has passenger cars) be one row of seating with an aisle to one side. With that in mind, I reduced the car width to equal that of G. This gives us a body that is 90″/7′-6″ wide.
B & C – This is blank for the On2 gage. Since this (I believe) is so the cars/loco will fit in a tunnel I guess this wasn’t needed for the Maine 2-foot gauge. Since my On18 gauge line’s (DRC&C) main purpose is to move coal from a mine to where it is processed – surely at some point it runs through a tunnel … this IS a model railroad after all. With that in mind I added B & C. The angle from C to B is approximately 55° on the S-7 Gage so I just used that to get close for even numbers for B&C.
D – This has to do with the height of the human body. For On3/On30 and On2 it is the same 2-1/2″ or 9ft. That will remain the same for On18
E – This is a mix of the size of the trucks, wheels, frame etc. I like the idea of the wheels being approximately the size of the wheels for On2. Let’s make it just a bit smaller – 1/8″ less then On2 – 2-3/8″ (80%)
G – On3 is 54″ over gage. On30 is 57″ over gauge. I’m going to set this for On18 at 15/16″ or 45″..

  • For On3, On30 and On2 this is 4-1/4″ or 17 ft.
  • For On3/On30 this is 1.42 x body width
  • for On2 this is 1.48 x body width
  • If we multiply then this 1.48 x 10′ (On18 body width) we get 14.8 ft. or 3.7″. Round it up to 3-3/4″ and we are done.
gagesSome .040 in. Styrene and I have my gage – along with a On30 gage that was made directly from the NMRA S-7 data.

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