A group of citizens led by Matthew Chapman pleaded their case to stop the REM in front of Quebec's Court of Appeal on Friday.
They are arguing for a constitutional right to a healthy environment, and say the construction to build the electric rail line will cause damage to greenspaces in Montreal.
"It is not too late to modify the project," said Chapman.
The group believes the REM could ultimately increase greenhouse gas emissions by encouraging urban sprawl on the South Shore and west of Montreal.
"Obviously the project is already underway. We're talking about looking at it carefully and saying what do we want to keep, what do we not want to keep. What makes sense for the climate, what does not make sense for the climate," said Chapman.
"If we build this project, the city will continue to sprawl westward and southward."
Chapman and five others say the public consultations held by the Caisse de Depot et de Placement du Quebec (CPDQ) were a charade where the agency refused to answer questions from the public.
They also argue the consultation process was pointless since the provincial government overrode the concerns brought up by Quebec's environmental review board, the BAPE.
On Friday morning lawyers for the government and CPDQ said the Quebec Court of Appeal has no jurisdiction to rule on the case.
Chapman's coalition said that wasn't so, and called on the Court of Appeal to force the new, legitimate public consultations.
"It's been 50 years we've been waiting for this project. Another year of consultations is not going to kill the project," said Chapman.
The case is now being deliberated by the Court of Appeals.
