Some Common Modelling Scale/Gauge Combinations
There is occasionally confusion between inch and foot designations, thus some modellers and publishers prefer On2.5 or On2 1/2, rather than On30, etc. Confusion aside, they refer to the same scale/gauge combination and in reality there is more opportunity for confusion between US and UK/European scales: HO/OO and O/O, etc.
While some items can be used for either scale (HO/OO or O/O), there is roughly a 20% size difference and care should when combining US and UK models in the same scene. For example, it would make sense to place a 7mm workman beside a 7mm tractor in a foreground scene, while using a 1/4" workman beside a 1/4" tractor further back in the scene to generate a sense of perspective.
Additional information, including clearances and track component dimensions, can be obtained from the NMRA Data Sheets and Recommended Practices.
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-14: (1:43.5) 7mm=1' on 14mm gauge track representing 2' 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
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 could operate on a layout using 9mm gauge track (although not necessarily at the same time), while HO/OO and On30 could operate on another using 16.5mm gauge track.