The Environment Minister John Gormley has warned the Department of Finance that his party will not accept the abandonment or delay of Metro North.
The project, which will cost an estimated €2.5bn, has been called into question after it was revealed that up to €15bn in cuts must be made between now and 2014.
The Labour Party said it would consider shelving the project if it got into power.
But Minister Gormley said his party would ensure the project goes ahead.
"Metro North will work. It will be hugely popular and it will reduce our CO2 emissions very significantly," he said.