There is T gauge now ( it runs on 3 mm track ) but it is so small that it lacks “realism” and power engines need much improvement .

From wikipedia :

T gauge (1:450 or 1:480) is a model railway scale with a track gauge of 3 mm (0.118 in), referred from "Three-millimeter gauge" or "Third of N scale".

One source if T trains :
https://www.tgauge.com/