Light rail

A tram/light rail system is the cheapest mass transit option for medium density suburbs, cheaper than freeways and high density underground rail.  Sydney, after getting rid of its tram network in the 50s, has started rebuilding a light rail network, as the blogger "My Ordinary Life" mentions:

The Sydney Light Rail has announced it is adding 90 services a week to its Central Station to Dulwich Hill line. Passenger numbers rose 60% in the last financial year. The line was extended to Dulwich Hill in 2014. 

This bodes well for the Randwick and CBD extensions currently under construction.

Light rail is nice: it runs like a tram on city streets, but becomes a train of sorts in the suburbs.  And the power it runs off can come from renewables, it produces no diesel fumes, and its stops (in the city) are close to where you want to go.