the Great Zig-Zag at Lithgow

One of the most notable railway engineering works in Australia is the Great Zig-Zag at Lithgow in the Blue Mountains. Built in the late 1860s and bypassed with the "10-tunnel deviation" opened just over 100 years ago, the Zig-Zag is now home to a museum railway operation that runs ex-Queensland Railways locomotives and carriages on the converted 3'6" gauge track.  Website

However, the best way to experience it is to walk up it from the bottom when trains aren't running (particularly in changeable weather which may require sheltering under one of the viaduct arches!). The old photos taken not long after it was built when the sides were still largely denuded of foliage better convey the experience of being there than recent photos.