Melbourne’s City Loop – when racing to catch a train from the lowest level of Parliament station, it feels like a journey to the centre of the earth. But how deep are the stations in reality?
The City Loop consists of four tunnels over two levels: platforms 1 and 2 on the upper deck serve the lines through Clifton Hill and Caulfield, while platforms 3 and 4 on the lower deck serve the lines that pass through Burnley and North Melbourne.
Unfortunately detailed diagrams of each City Loop station aren’t accessible online, which rules out the obvious way to read the depth measurement. Firing up the GPS on your phone could be another way, but getting a fix on the satellites from underground is impossible, and even if you could – the altitude accuracy of a consumer GPS device is about +/- 15 meters!
I ended up turning to another set of diagrams – the railway issued ‘Grades and Curves’ book that shows the general alignment and elevation for each of the City Loop tunnels. (the full set of diagrams can be found at Vicsig)
The diagrams in the ‘Grades and Curves’ book are used to assist train drivers in learning the routes they will be driving along – trains don’t accelerate and stop like a car, so knowing about sharp curves, long inclines and steep descents is essential to keep the train under control.
Unfortunately the Grades and Curves diagrams only include elevation in metres above the Low Water Mark – a datum set as the level reached by seawater at low tide – and not in metres below the surface!
To determine the ground level elevation of each station, I turned to the survey marks database operated by Land Victoria. It gives the elevation of thousands of survey points around Australia, all given in relation of the Australian Height Datum (AHD) – a height set to the mean sea level for 1966-1968 calculated at thirty tide gauges around the coast of the Australian continent.
With the depth of each platform given by the Grades and Curves diagrams, the ground level elevation from survey marks, and the the rest is just simple arithmetic.
For Flagstaff station the Grades and Curves diagram states the upper level is at +8 metres LWM, and the lower level is at -1 metres LWM. William Street is at the crest of the hill, so survey mark ‘MELBOURNE NORTH PM 50 MURLA’ (#308300500) on nearby Little Lonsdale Street can be used, located at a height of 28.446 metres above AHD.
|Surface level||+28.446 metres AHD|
|Rail level, upper platforms||+8 metres LWM|
|Depth, upper platforms||20.5 metres|
|Separation between platforms||9 metres|
|Rail level, lower platforms||-1 metres LWM|
|Depth, lower platforms||29.5 metres|
Melbourne Central Station
For Melbourne Central station the Grades and Curves diagram states the upper level is at +2 metres LWM, and the lower level is at -7 metres LWM. La Trobe Street is a complicating factor – the Swanston Street end being higher than than that at Elizabeth Street – so I looked at two survey marks – ‘MMB 380’ (#450003801) on the Elizabeth Street corner is at 13.450 metres above AHD, while mark ‘MMB 388’ (#450003881) is located at a height of 22.370 metres above AHD.
|Surface level||+13.450 / +22.370 metres AHD|
|Rail level, upper platforms||+2 metres LWM|
|Depth, upper platforms||11.5 / 20.5 metres|
|Separation between platforms||9 metres|
|Rail level, lower platforms||-7 metres LWM|
|Depth, lower platforms||20.5 / 29.5 metres|
For Parliament station the Grades and Curves diagram states the upper level is at +7 metres LWM, and the lower level is at -3 metres LWM. Spring Street is relativity flat atop the station, so I used survey mark ‘MELBOURNE NORTH PM 16 MURLA’ (#308300160) on Lonsdale Street, located at a height of 36.746 metres above AHD.
|Surface level||+36.746 metres AHD|
|Rail level, upper platforms||+7 metres LWM|
|Depth, upper platforms||29.5 metres|
|Separation between platforms||10 metres|
|Rail level, lower platforms||-3 metres LWM|
|Depth, lower platforms||39.5 metres|
So what have I discovered?
- Parliament station is the deepest below ground (39.5 metres) on the City Loop
- Melbourne Central is the furthest station below sea level, but is the closest to the surface (between 20.5 and 29.5 metres) due to Elizabeth Street being in a valley
- Flagstaff station is the highest station above sea level, but is the second deepest (29.5 metres) due to the location atop a hill
Checking my answers
After doing the maths myself, I remembered that the Metropolitan Transport Authority put together a booklet of City Loop facts and figures in the mid-1980s.
It quotes the following depths:
- Parliament station: 40 metres
- Flagstaff station: 32 metres
- Melbourne Central station: 22 / 29 metres
I was out by 3 metres for the depot of Flagstaff station, but for the other locations, my roundabout way of determining the City Loop depth was reasonably accurate!
An alternate method
Another method to measure the depth of an underground station is to measure the height of an escalator step, count the total number of steps between platform level and the surface, and multiply them together to get the total height.