West Australian Premier Colin Barnett admits some voters may not believe the Liberals' Thornlie train line election pledge will be delivered if the party wins a third term, conceding previous rail promises have been broken.
The state government has matched part of the opposition's Metronet rail plan in promising a link from Thornlie to Cockburn in Perth's southern suburbs, with completion slated for 2023.
Labor has pointed to the Liberals' track record of abandoned train projects, including a link to Ellenbrook in the city's northeastern fringe, promised in 2008.
"The public would be cynical about the Liberal party's commitment today," opposition transport spokeswoman Rita Saffioti told ABC radio.
Mr Barnett said Labor voters may be sceptical about the $520 million pledge, but the party had been considering the important project, which would be the city's first cross-suburbs train line, connecting with the existing Mandurah and Armadale lines, for decades.
It would remove the need for many people to travel into the CBD and slash the time of their commute, he said.
Mr Barnett said his government was delivering one of two major 2013 election promises, a rail line from the airport to Forrestfield, but the MAX light rail proposal had been shelved, saying City of Perth opposition to it going through the CBD had proved a major obstacle.
"Call it a broken promise if you like, I concede that was," he said, adding MAX would not be revived during a third-term Liberal government.
"At some stage in the future, you'll probably see a tunnel line to Morley, branching out to Ellenbrook and to Wanneroo. That is long-term planning."
Ms Saffioti said the Thornlie project was much needed and long overdue, but the Liberal plan would take too long to deliver.
"The local member said planning was nearly completed in 2014," she said.
