Do you mean the S01 track standard Philip? If so it gives the recommended dimensions for track and wheel sizes that allow G3 locos and stock to run on any compliant G3 track. This is important in G3 (as opposed to most MES tracks) as we have turnouts (points) that require certain clearances.
IanT
This thread was discussing modelling the British Broad Gauge, the 7' gauge promoted by Brunel and implemented by the Great Western Railway until the 1890s. The G3 Society standards posted by AndyB are for the standard gauge 4' 8 1/2".the thread entitled "BG13.5 Broad Gauge in Gauge 3" a poster
This thread was discussing modelling the British Broad Gauge, the 7' gauge promoted by Brunel and implemented by the Great Western Railway until the 1890s. The G3 Society standards posted by AndyB are for the standard gauge 4' 8 1/2".