William Street tunnel

The William Street tunnel is a railway tunnel under the central business district of Perth, Western Australia. It was built between 2004 and 2007 as part of the construction of the Mandurah line, which it connects with the Yanchep line, running under William Street for much of its length. It consists of 770 metres (2,530 ft) of twin bored tunnels, a cut-and-cover tunnel at either end, and two stations: Perth Underground and Elizabeth Quay.

The Mandurah line was initially proposed to branch off the Armadale line. After a more direct route was chosen in 2001, a tunnel under the central business district became necessary to connect the Mandurah line to the rest of the network. The construction of the Mandurah line was divided into eight contract packages; the William Street tunnel was part of Package F, also known as the City Project, the contract for which was awarded to Leighton–Kumagai Gumi in February 2004 for A$324.5 million. Preliminary works began the same month, and tunnelling began in October 2005, starting from Elizabeth Quay station and heading north. Boring for the first tunnel was completed in June 2006, after which the tunnel boring machine was transported back to Elizabeth Quay to dig the second, which was completed in October 2006.

Construction was disrupted by industrial action, which culminated in the prosecution of 107 workers for illegally striking in February and March 2006 following the issuance of a strike ban by the Australian Industrial Relations Commission. The strikes, along with complications involving heritage protection at Perth Underground station, engineering challenges on the Swan River foreshore, and contract disputes, resulted in the tunnel's opening being delayed beyond its 2006 deadline. The first train entered the tunnel in August 2007 and it opened to passengers on 15 October, ahead of the rest of the Mandurah line's opening on 23 December. The final cost, after contract disputes were settled, was $439.3 million.