When TT gauge model railways were developed for British prototypes, in order to fit the small British prototypes, the scale was enlarged but without altering the 12mm gauge.
By far the more common of these is 14.2 mm gauge track, which is accurate.
Some modellers choose to use slightly narrower 13.5 mm track due to the necessary oversize motion of outside-cylindered steam locomotives.
British TT, also known as TT3, was pioneered by Triang Railways as ready-to-run models.
The 3mm Society caters for all modellers of 3 mm scale.