Building a new parallel track will never happen, so let's use tech to build a few bypasses for the regular train so the fast non-stop train can whiz past.
The normal all-stops train starts off. Later the fast train sets off. The normal train is told to pull into one of several bypasses to let the fast train by. Then the normal train carries on. Currently a train leaves Wollongong at 6:57 and arrives at Central at 8:25, so about 1.5 hours. It stops at 7 stations. If if stopped at Central only it would run into an earlier train, so that single train would need to pull over to let the fast train pass.