There is no "standard" narrow gauge: a variety of track gauges have been used for smaller-than-standard-gauge railways around the world. But threeof the most common are reasonably close to each other:
---42 inch gauge (South Africa, other African systems, New Zealand, Newfoundland...)
---Metre gauge (LOTS of "third world" countries; narrow gauge in continental Europe (including the prototypes for LGB))
---36 inch gauge (many North American narrow gauge systems (incl. Colorado and WP&Y), Irish narrow gauge...)
These are close enough that they could be treated as compatible in modelling. Using HO track (16.5mm gauge), we would model
---42 inch gauge rolling stock in 1/65 scale (almost S-scale!)
---metre gauge in 1/61 scale
---36 inch gauge in 1/55th scale.
(Scales rounded out to have two-digit denominators.)
If the demand for assorted narrow gauge model railroad stuff was great enough to attract commercial manufacturers, I would suggest that "generic" items (structures, figures, diesel locomotives like the Alco Dl-531 and Dl-535 that were built for systems of several track gauges) be modelled in the middle, 1/61 scale. They will be a little small on a lay-out representing South African Railways and a bit big on layouts representing Colorado narrow gauge, but maybe within a believable range. For example, a figure representing a six foot tall man in metre gauge would represent a 6'5" man -- tall, but well within the range of pro-basketball players! -- on a 42 inch gauge layout, and a 5'5" man on a 36 inch gauge layout.
---42 inch gauge (South Africa, other African systems, New Zealand, Newfoundland...)
---Metre gauge (LOTS of "third world" countries; narrow gauge in continental Europe (including the prototypes for LGB))
---36 inch gauge (many North American narrow gauge systems (incl. Colorado and WP&Y), Irish narrow gauge...)
These are close enough that they could be treated as compatible in modelling. Using HO track (16.5mm gauge), we would model
---42 inch gauge rolling stock in 1/65 scale (almost S-scale!)
---metre gauge in 1/61 scale
---36 inch gauge in 1/55th scale.
(Scales rounded out to have two-digit denominators.)
If the demand for assorted narrow gauge model railroad stuff was great enough to attract commercial manufacturers, I would suggest that "generic" items (structures, figures, diesel locomotives like the Alco Dl-531 and Dl-535 that were built for systems of several track gauges) be modelled in the middle, 1/61 scale. They will be a little small on a lay-out representing South African Railways and a bit big on layouts representing Colorado narrow gauge, but maybe within a believable range. For example, a figure representing a six foot tall man in metre gauge would represent a 6'5" man -- tall, but well within the range of pro-basketball players! -- on a 42 inch gauge layout, and a 5'5" man on a 36 inch gauge layout.
