This is the "island platform" at St James railway station. The rail tracks are to the extreme left and right of the picture beyond the colums.
St. James was originally intended to be a major interchange station. As a result, St James has several abandoned tunnels, one of which has flooded and produced an underground lake. St. James station was designed to support four tunnels, but only two were ever constructed. The remaining two are stubs, which lead some way off from the station but then abruptly end. The intention behind this was that if they later decided to use these tunnels, they could extend them without causing interference with St. James station, thus permitting it to remain in operation.
Despite not going anywhere, they have still seen a significant amount of use over the years. This has included use as a mushroom farm and a World War II bomb shelter.
The two disused tunnels lead off both to the north and south of St. James station, in between the two tunnels currently in use. The platform of St. James Station is in fact two separate island platforms, with a space between them for the tunnel tracks; but this space has been covered over with a false floor, giving the appearance of a single platform.
For those interested, here is a detailed history of the St James tunnels. Please note that the Australian Railway Historical Society is no longer running tours of the tunnels.