WA Labor Leader Mark McGowan says, if elected, he will build the multibillion-dollar Metronet rail system that connects Thornlie to the Mandurah line.
In July, the RAC called on the Federal Government to build a line between Thornlie and Cockburn at a cost of more than $300 million.
Premier Colin Barnett also said at the time it was a priority project.
The line would connect to the Mandurah line, but no additional stations have been suggested.
Mr McGowan did not specify how much it would cost or when it could be built, but said it was a requirement for Perth.
“One of the things about the Metronet plan is that it has east-west connections,” he said at Thornlie railway station.
“That is very important, otherwise management of traffic and people of Perth Stadium will be very, very difficult.
“It is needed and it is something essential for the long-term future for the City, which I will fulfil if I am elected premier.”
Mr McGowan said one of the reasons why a rail link between Thornlie and the Mandurah rail line was so important was because of the potential congestion caused by the number of people coming out of the new Perth Stadium in Burswood.
Metronet was first unveiled before the 2013 state election. The next state election is expected to be called in early 2017.
Mr Barnett said Mr McGowan had been consistently opposed to the new stadium at Burswood.
“We are building a new six-platform rail station at the new stadium that would take trains from all parts from Perth,” he said.
“It’s logical the northern suburbs railway will go further north.
“It’s logical other railways will be built.”