2 mm/foot scale isn't N gauge....and doesn't 'use' 9 mm track gauge, as such......unless the builder really wants to.
but, then one gets into the 'finescale' end..for example...4 mm/foot scale..ie 1:76-as-near-as-dammit...called ''OO'' gauge when using a track gauge of 16.5mm......becomes ''EM'' when using a more closer-to-scale 18.2mm gauge of track....[it confused me, because when I started, EM gauge track was at 18mm.......someone sneaked behind my back and changed it]...then there's ''P4''..or even once, 'S4'..which uses the same scale, but a gauge of 18.83mm.
O gauge also has its problems...especially with the USA having a 'coarse' O scale....plus there is ''Scaleseven'....
Actually...track 'gauge' is really rather an irrelevance...if you see what I mean......
because, the actual 'gauge..whether prototype or model, is really only an approximation, since it will vary according to circumstances....[gauge widening on curves,for example?]
With 4mm, and other scales, teh 'finescale' bit really refers to...in PW terms, the tolerances regarding flangeways, checkrail gauge, and wheelsets.....especially wheel profiles.
also, it seems, 'finescale' now refers to 'fidelity to protoype'......regarding appearances,....
and operations.....including infrastructure.
I pity those modelling in N .
especially with the European propensity for changing the actual scale ratio to suit ???