O Scale is 1:48 proportion is USA (see not below)

Prototype Gauge 4'8 1/2" = 1.177083333

NMRA Standard Gauge is min 1.25" to max 1.285" (S-1.2 Standards for Scale Models)

On30 uses HO scale wheel and track geometries, as specified in S-3.2 and S-4.2.
track gauge is 0.650" or 16.5 mm
An HO Scale 33" wheel actual diameter is 0.379". This would be 18.19 inches.

N Scale track gauge (9 mm or 0.355") would be 17 inches
A N Scale 33" wheel actual diameter is 0.206". This would be 9.9 inches.

Rail Sizes

Rail Size	Scale Height	Proto Size
Code 55	2.64"	20 lb
Code 70	3.36"	35 lb
Code 83	3.98"	55 lb
Code 100	4.8"	75 lb
Code 125	6.0"	100 lb / 110 lb
Code 148	7.1"	130 lb

In Britain, O Scale is 1:45, 7mm = 1 foot. 4'8 1/2" is 1.256 inches. A British O scale model would be 6.7% larger than the same model at 1/48 scale.
1:45 scale would be 6.714mm = 1 foot, so it's still a compromise.

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O Scale Kadee couplers 0.781" from top of rail to top of draft gear box

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O Scale Track Dimensions
O Scale Ma and Pa Flat Car Drawings
O Scale Plymouth 20 Ton Locomotive

References
New Haven 1937 AAR Box Car
The O Scale Resource Magazine
Colorado O Scale Modelers
O Scale Trains Magazine
Proto 48

Products
Atlas O Scale Trucks and Couplers
Athearn O Scale Archbar trucks (discontinued)
Athearn O Scale Andrews trucks (discontinued)
Athearn O Scale Bettendorf trucks (discontinued)
Kadee O Scale wheels and trucks
Right-o-way rail, spikes
Midwest Products Cork Roadbed 3' (25 pieces) (out of production)
Woodland Scenics O Scale Products
Woodland Scenics O Scale "Trackbed"
Wiseman Model Services Trucks and other parts
All Nation Line
American Scale Models O Scale
La Belle Woodworking
Wiseman Model Services - Link and Pin Couplers
Kadee 815 On30 Multi-Purpose Coupler Height Gauge with On3 Coupler
Kadee 300 On30 Scale 24" Diameter Ribbed Back Griffin Wheels
N / HO / O scale Industrial Modular Wall Systems

On30 Products
Kitwood Hill Models (UK)
Micro Engineering has On30 track
Bachmann 71-025 Quarry Hunslet 0-4-0ST 'Alice' Dinorwic Quarry Red Steam Locomotive
Foggy Mountain Models
Davenport 0-4-0 Gas Mechanical Coming soon(?)



Specific parts
All Nation AAR Type A Truck ASSEMBLED with 40″ Wheels (Pair) PN#285AN
All Nation Doodlebug Power Truck Chain Drive & Trailing Truck � 12VPittman for (e.g.) Gas Electric, Doodlebug or WindSplitter 3D Printed Sides PN#469AN
O SCALE BRASS WESTINGHOUSE FREIGHT CAR A-B BRAKE SET
O SCALE On3 OR On30 WESTINGHOUSE K TYPE BRAKE CYLINDERS
O SCALE WORKING LINK & PIN COUPLERS
On3/On30 LINK & PIN STYLE COUPLERS, FIT KADEE #4 STYLE COUPLER BOX
On3/On30 LONG SHANK LINK & PIN COUPLERS FOR KADEE #5,#26 STYLE BOX
On3/On30 MEDIUM SHANK LINK & PIN COUPLERS FOR KADEE #5 STYLE BOX_#148 HO Scale 140-Series Whisker� Metal Couplers with Gearboxes - Medium (9/32") Centerset Shank