>>1446589So, first the easy part, the underground suburban loop is literally a massive boondoggle project, it would monopolize funding and paralize all other improvement projects. It's only in very exceptional situations that one may do one massive project that would solve a big problem all at once. The underground suburban loop is more than massive, and it wouldn't solve much.
Instead, there's a lot of disused connections (I would assume freight connections) that form almost a complete loop around the inner city. Some parts have been removed and turned into linear parks (which are urbanistic poison btw).
So first off, I'd suggest the pic related loop line, just outside the city center, where density is still notable and demand for this justified, and also where it would require no massive construction projects. It could also be put into service gradually. The parts that have been turned into parks could be re-built as elevated rail line, still cheaper than underground.
Now about Doncaster, since the area is mostly urbanized building a new rail line would be expensive. What might be a possibility is to build a fast tram line to Mont Albert station. This is a distance of less than 5km, which at 20km/h commercial speed would be 15 minutes, and at 30km/h (assuming somewhat long stretches without stops) 10 minutes. If this is synched with the trains, it would be reasonably convenient.
