Great thinking ahead, except plans changed and when (decades later) they eventually decided to build the Eastern Suburbs line, they chose a completely different alignment. So they just had these stub tunnels sitting underground, disused. During World War II they were converted to being an air raid shelter. They've seen some other random uses over the years, such as being used to film TV shows, part of them is used as a maintenance train siding, etc. Now the government is seeking proposals for the private sector to redevelop them into something useful.
When the Eastern Suburbs line was finally built on a different alignment in the 1970s, they did something similar – it has two underground platforms at Central Station, but they actually built four underground platforms. Two of them active, the other two with stub tunnels not going anywhere. Once again, this was to support another planned railway line, the Southern Suburbs line, which has never built either. For many years, the never-used platforms were used as archival document storage for the government rail authority. Now, Sydney is building a new metro line. The Metro line is not going to use these platforms, it is using newly constructed underground platforms. But these never-used platforms are going to be converted into utility rooms for the Metro line.
Similarly, at Redfern station, there is a big pit in the ground which holds half-constructed platforms for the Southern Suburbs line. I doubt those platforms will ever be used for that purpose.
So anyway, planning ahead is a great idea, but often it just turns into a waste.