There isn't one. There are no direct trains from Canberra to Melbourne.
The options are:
- Train from Canberra to Goulburn, then train to Melbourne
- Bus from Canberra to Yass Junction, then Countrylink train.
- Bus from Canberra to Wodonga, then V/Line train.
Check out http://www.railpage.org.au/tt/
Cheers
David