Otford Tunnel - Helensburgh
Otford tunnel is one of a series of 7 single line tunnels built around Helensburgh in 1888, and formed the gateway to what was the old South Coast Railway, joining Wollongong to Sydney.
This tunnel has a heritage listing due to its significant structural engineering design and, back in its day, was one of the longest and steepest single line tunnels to have be built at a length of over 1.6km.