Nick Ridgway
0.1mm represents 1/40 of a foot in 4mm scale, which is a little over 1/4 of an inch. This is within the normally-experienced tolerances on prototype track: “1/4 tight” has been deployed on tube lines to stop the motor bogies from thrashing about when accelerating, for example. “5/8 wide” is sometimes deployed on sharp curves to reduce the prospects of flange and rail wearing each other out; I’ve done it myself in 12in scale.
Bear also in mind that EM gauge track is a scale 2&1/2in narrow compared to the prototype. Where does it end (rhetorical question – NNTR)?