Greg Sutherland and others have often commented on the practice of 'easing the gauge' on curves.

Richard Clark has h=kindly provided details in the attachment ...

Thanks Richard!.

===

Sydney Tramways had a standard for Gauges Grooves on Tramway Curves, attached, more than 120 years ago.

In their wisdom grooved rail, then called girder rail, not to be used for under 100ft (30m) radius curves.
The groove is increased from 7/8in (22mm) to 1 11/16in (43mm) for tight curves.

The tramway operations included fixed truck coupled cars such as E cars and K cars.

Rick C
A registered professional engineer