CityRail is part of RailCorp, therefore you should visit the RailCorp website, and look in the careers tab, you will find all the positions that are current vacant.( Here's a link I prepared earlier
Alternatively, if you wanted a job with RailCorp you can also try the NSW Government jobs portal, clicking on RailCorp as the agency;https://jobs.nsw.gov.au/Start.asp
At the moment, a great "entry level" position currently being advertised is CSA, ( think station staff) (closing 28/11/2008, so be quick!!). Once you get accepted, your foot is in the door for countless other opportunities.
Edited: Removed syntax error, corrected spelling