I just had bit of knowledge of the a bus system used in Adelaide called the O'Bahn, which is a bus way system guided on dedicated tracks and can be used onto the roads where it have a dedicated lane normally bypass traffic. The buses running express can run at high speeds and higher frequency and capacity. This kind of system could be implemented for Doncaster which already has a DART (Doncaster Area Rapid Transit) which already has four frequent bus route linking Doncaster to the CBD but does get stuck at intersections and can be much slower due the Eastern Freeway clogging up with traffic.
I believe it has some merits, and should be investigated and planned to parts of Melbourne like Doncaster or Rowville. The capacity would be enough if more expressing dedicated O'Baun system was used in areas where new rail extensions are not viable.