The line was originally only a single track, and featured Western Australia's first and, until 1990, only railway tunnel. Due to ventilation problems, in 1942 the tunnel was the cause of a runaway train crash, prompting calls for a fix. As traffic increased the newer route was duplicated in the 1930s, with the Tunnel Junction signal box established to manage traffic through the still single-track tunnel. A single-track diversion of the tunnel was completed on 25 November 1945, allowing trains heading uphill towards the east to avoid the tunnel and providing double-track operation along the entire line.