The last sector before we reach metropolitan Sydney presents few design difficulties, or at least no more so than the adjacent Hume Highway. A couple of bridges and moderately deep cuttings are all that is needed to bring the high-speed railway back to the Southwest Rail Corridor, where the prevailing radius is about 1800m.
In this 20-kilometre stretch, the line enters a sustained gentle-to-moderate descent from the Southern Highlands down into the Sydney Basin, dropping over 300m in altitude. There aren’t a lot of plausible corridor options here; the Hume Highway already occupies by far the best one, and it is of sufficient straightness and flatness for a high-speed rail alignment.
As we get closer to Sydney, the Southern Highlands constricts down to a narrow spit of cleared land nestled between the Blue Mountains on the west and the Upper Nepean gorge on the east, and almost all of the transport infrastructure squeezes into a narrow 500m-wide band between Yerrinbool and Bargo. This limits the room available for deviations but also gives us a choice of several existing corridors to utilise.
The geography between Moss Vale and Mittagong presents a significant barrier to infrastructure development, both due to the built up areas of Bowral and Mittagong and also the imposing topography of Mt Gibraltar. For this reason, re-use of the existing corridor is the best and probably only option here.
Now we are really getting into the heart of the Southern Highlands, with its beautiful rolling pastures, cute hamlets, manicured golf courses, and of course the Shoalhaven Escarpment on the southeastern side. The existing railway between Bundanoon and Moss Vale is pretty windy, so we will need to build a completely new track through moderately difficult terrain.
The existing track between Wingello and Bundanoon winds its way along the northwestern escarpment of the Shoalhaven, making for a dramatic but winding railway. Unfortunately there are only limited opportunities to re-use the existing alignment, so a substantially new alignment will be needed.
The Marulan sector consists of about half existing alignment and half new alignment around Uringalla. The country is undulating, meaning that significant earthworks and possibly some tunnelling and structures may be required.
This sector involves some difficult terrain, with the Wollondilly River winding between hilly country. The eastern part of the alignment approaching Marulan is unproblematic; curves are minimal and there is even a former trackbed on the north side of the tracks that can be reutilised for most of the length.
The existing alignment out of Goulburn has a minimum radius of 600m, yet it should be possible to increase to 1000m or possibly 1500m with only small modifications to the existing corridor. Geography, other transport corridors and existing development impose substantial constraints beyond this; any faster deviation will be very expensive and should therefore be deferred until higher-speed operations are required.
North of Bungendore, the line encounters rolling countryside with a few substantial hills that must ideally be avoided. There are fewer sections of the existing alignment with minimum radius under 1000m (the smallest radius at which 200km/h is possible with tilting trains). We are therefore able to use the existing…