New South Wales Transport Museum stages this event over the 3 days of Queens Birthday Weekend in June. This year's event added steam boats to the mix of buses and old trains.
To start with a bang, the first event on Saturday is the Great Train Race, wherein 2 (or 3) trains "race" from Central to Strathfield station. After that event, there are scheduled shuttle services from Central, mostly to Hurstville, but the rail motors had 2 unusual routes - one via Olympic Park station, and the other along the freight lines normally inaccessible to mere mortals.
Information on Garratt locomotives:- https://en.wikipedia.org/wiki/Garratt
This particular Garratt:- https://en.wikipedia.org/wiki/6029