The Land Transport Authority (LTA) met nature groups on Tuesday evening - the first time it has done so - after concerns were raised over the environmental impact of the proposed Cross Island MRT Line.
Preliminary plans have a portion of the line passing underground through the nature reserve in the central catchment area.
The groups have offered to conduct a study of the line's impact on the flora and fauna of the nature reserve, which is expected to take several months.
LTA chief executive Chew Hock Yong said the authority will decide on its next step and how it should conduct an environmental impact assessment after taking in findings from the study.
He noted that the actual corridor of the line has not been decided upon, as planning is still at a very early stage.
The 50km Cross Island Line, which will run from Changi to Jurong, is slated for completion in 2030.