I remember this from my childhood. An oval of track with a tunnel/storage shed where the train was kept when not in use. Petrol-powered steam looking loco and 3 or 4 open seat 'carriages'. Probably around 3 ft gauge. May have a picture in my parents photos somewhere...
Off topic but sort of related there were articles in the North Shore Times and/or Hornsby Advocate in the early 70s about a proposed 5" railway to run from the animal park/visitor centre down to Bobbin Head. Came from a person or club in St Ives. Nothing ever happened as far as I know.