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.
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 Size	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 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