Additional information, including clearances and track component dimensions, can be obtained from the NMRA Data Sheets and Recommended Practices.

Some Common Scale/Gauge Combinations

Interestingly, a few model layouts have been built with loading gauges (clearances) that permit multiple scale operation, albeit with unrealisticly sized buildings, etc., for at least one scale. Thus N, 009 and HOn30 trains might operate on a layout, while HO/OO and On30 might operate on another.

N
(1:160) .075"/1.9mm=1' on 9mm gauge track representing 4'8.5" standard gauge
HOn2.5/HOn30
(1:87) 3.5mm=1' on 9mm gauge track representing 2' - 2' 6" gauge
009
(1:76) 4mm=1' on 9mm gauge track representing 1'10" - 2' 6" gauge
HOn3
(1:87) 3.5mm=1' on 10.5mm gauge track representing 3' gauge
HO
(1:87/) 3.5mm=1' on 16.5mm gauge track representing 4'8.5" standard gauge
OO
(1:76) 4mm=1' on 16.5mm gauge track representing 4'8.5" standard gauge
Sn3
(1:64) 3/16"=1' on 14mm gauge track representing 3' gauge
Sn3 1//2
(1:64) 3/16"=1' on 16.5mm gauge track representing 3'6" gauge
On2
(1:48) 1/4"=1' on 12.5mm gauge track representing 2' gauge
On2.5/On30
(1:48) 1/4"=1' on 16.5mm gauge track representing 2'6" gauge
O-16.5
(1:43.5) 7mm=1' on 16.5mm gauge track representing 1'10" - 2'6" gauge
On3
(1:48) 1/4"=1' on 19mm gauge track representing 3' gauge
O (US)
(1:48) 1/4"=1' on 32mm gauge track representing 4'8.5" standard gauge
O (UK)
(1:43.5) 7mm=1' on 32mm gauge track representing 4'8.5" standard gauge
SM32
(1:19) 16mm=1' on 32mm gauge track representing 2' gauge
G
(1:24 or 1:22.5) 1/2"=1' or 13.5mm=1' on 45mm gauge track representing ~3' gauge