Now this may seem like an obvious idea, but I have yet to hear it suggested.
Could not the Melbourne public transport authorities - or those of any city on the globe - provide the option of a Green Ticket?
All they would have to do is divide their energy use/carbon emissions by the number of tickets sold, and arrive at some sort of figure for the carbon cost of each ticket. Then, with each purchase of a green ticket, they would undertake to either offset [in the case of buses etc.] or purchase accredited green energy [trains, trams etc] to that arrived at value.
The Green Ticket would be a little more expensive, but every purchase would reduce greenhouse gas emissions. And the government might even see fit to cover that extra expense itself…