The eastern suburbs line, running between Bondi Junction and the city, was chosen as the first train route to use the card because it is relatively contained.

Commuters will initially be able to use the card to get on and off at any station on the city circle and on the eastern suburbs line, but not beyond.

The philosophy behind starting the trial on a narrow section of the train network is the same as the thinking behind trialling the Opal card on one ferry route - the Neutral Bay service - which started late last year.

Since the ferry trial began in November, 533 commuters have signed up to use the card, modelled on London's Oyster card.

In the week to January 21, card readers on the Neutral Bay route recorded 482 journeys.

''We are seeing the benefits of a small, controlled trial and progressive roll out - one of the reasons the Neutral Bay route was chosen,'' a spokesman for Transport for NSW said.

''For example, we have been able to contact customers when they forget to tap off,'' the spokesman said.

''It's also clear … that most customers who interchange onto other modes are likely to wait until Opal is available on that mode before they use it.''

Manly is the next ferry route to gain access to the Opal, which should be in place across all trains, buses and ferries by the end of 2014.