WA Labor's biggest election promise - a massive expansion of Perth's rail system will cost $2.5 billion and include a $416 million Commonwealth cheque dependent on the generosity of the Turnbull government.

Opposition Leader Mark McGowan, who is ahead in the polls, is adamant he will get the federal cash if elected next month and his flagship project will not have a huge funding hole. Labor is betting on the Commonwealth funds arguing that because it will dump government’s flagship project, the…