A flying junction is a railway junction at which one or more diverging or converging tracks in a multiple-track route cross other tracks on the route by bridge or bridges to avoid conflict with other train movements. A more technical term is 'grade-separated junction'. A burrowing junction or dive-under occurs where the diverging line passes below the main line.
When Sydney's Central Station was extended in the early 1920s, it included electrification of aa suburban lines. The original proposal for electrification was for the North Shore line, from Hornsby to Milsons Point via the Harbour Bridge, to be a separate line which could be electrified without impact on the remainder of the rail system. However, due to the necessity of building the City Underground Railway, not to mention the expansion of the Illawarra and Bankstown lines, the program was altered in order that the electrification could be linked with these proposed expansions.
The existing Sydney Terminal would remain untouched. From Well Street, Redfern eight tracks would continue as the City Railway whilst four would carry the country and interstate trains to the Sydney Terminal. An above ground station would include a link to allow the transfer of passengers and baggage to the Sydney Terminal. A new terminal would be added at the station's eastern end through which suburban trains from the south could enter and exit the lines of the city circle.
Additional work to accommodate the electrification and expansion of the city and suburban lines was proposed and subsequently implemented by the Railways' Chieg Engineer, JCC Bradfield. These included extensions to the Cleveland Street Bridge, along a series of flyovers, which were realised in the flying junctions at the southern end of Central Station's suburban platforms between the Cleveland Street bridge and Central Station.
The Devonshire Street subway and Devonshire Street wall were extended through the new suburban section. A new viaduct along Elizabeth Street included new bridges over Eddy Avenue, Campbell Street and Hay Street and a new retaining wall along Elizabeth Street were also part of Bradfield's plan.
The flying junctions under construction, seen from the south end of the Central Station site, the supports for the flying junctions are advanced as shown in this photograh taken 7th October 1925.
The four island platforms allow eight trains to use the station, four trains in each direction. South of these new platforms are the flying junctions, allowing the four southbound tracks to pass beneath the northbound tracks with a series of diamond crossings allowing trains to cross lines without impeding trains traveling in the opposite direction. They allow trains approaching Central from Redfern to change tracks before they reach their designated platform. Bradfield had other flying junctions planned for around the Sydney suburban railway system, but they were never built.
Located between Redfern and Central stations are two dives, which take Illawarra intercity trains from one side of the railway lines to the other. The down dive has a 286.7 metres long tunnel, the up dive tunnel (above is 241 metres in length. These dives were originally used to allow rail access between the two sections of Eveleigh Workshops which are located on either side of the through lines.
There are a number of other dives on the Sydney railway ststem. On the Main Western Line there are also two dives, one between Parramatta and Westmead and the other between Seven Hills and Blacktown, which allow trains to pass from one side of the 4 track line to the other.
Sydney's flying junctions track diagram
Driver's view from the lower level
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