I know that Dreamworld in QLD had a single track loop that runs around the park (similar to Disneyland)— they had a large crash there in 1994(?) (day after I went there!). There was also a small steam railway at "Timbertown" at Wauchope in NSW— don't know if this still exists. The small park thingy next to the Port Arthur site in Tassie also has a railway in it.
Where else?