|railway = narrow_gauge|
|Passenger or freight trains on narrower tracks than the standard gauge for the country or state.|
|Rendering in OSM Carto|
|Used on these elements|
|Tools for this tag|
Just like there isn't a railway=broad or railway=standard, there should be no railway=narrow_gauge. Most people, except train aficionados, would not even notice if a railway line is say, 1435 or 1000 mm wide. The fact that it is narrow is simply a less important fact and should go into its own tag, gauge=*, instead.
Likewise, narrow gauge railways can carry most types of service, like mainline passenger, goods or industrial railways, so there is practically no difference of "importance" either.
This tag appeared in the OSM database in 2008 and has had a linear growth since then. Yet there is still only 52000 occurrences as of 2022. Out of those, 83% is already tagged with some value of gauge=*