It seems that almost everything other than trains uses "standard" scales. The only easy scales in use for trains are 1/32 (Gauge 1), 1/48 (O - North America only) and 1/64 (S). No 1/72, 1/96, 1/144, etc.
In the case of N, it's because it wasn't made to be a specific scale. Unlike older scales like O and HO (but like Z would later be), the scale ratio for N was determined by measuring Arnold Rapido models rather than the other way round.