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.