The alternative alignment that routes the Cross Island Line (CRL) around the Central Catchment Nature Reserve (CCNR) could add about $2 billion to the rail project, the Land Transport Authority (LTA) has revealed.
This 9km-long "skirting alignment" will require longer tunnels and extra ventilation facilities, it said, compared with the 4km direct route, of which 2km will cut through Singapore's largest nature reserve.
"Besides land and home acquisitions that could affect families, the extra works could incur $2 billion more in expenditure," said LTA chief executive Chew Men Leong.
In a letter to The Straits Times Forum page, signed by Mr Chew and published today, the LTA reiterated that the Government was studying both alignment options and had not made a decision.
Since a report on the environmental impact of site investigation works for the project was released recently, green groups have been lobbying hard for the CRL - a 50km MRT line that will span Jurong to Changi - to be built around the reserve, instead of through it.
However, this could entail land acquisition, as the MRT tunnels would pass through homes, businesses and buildings. Residents in Upper Thomson who could potentially be affected have called for the line to go through the reserve.
Mr Chew said the Government will consider all factors, including the engineering feasibility of both alignments, distance and travel time for commuters, cost to tax payers, and the impact on the CCNR and on businesses and families.
Details on the CRL, such as the number of stations and the construction cost, have not been announced.
Green groups conceded that the extra $2 billion and the impact on residents are significant, but called for deeper thought on the issue.
Dr Shawn Lum, president of the Nature Society, asked: "What is the long-term cost to Singapore of potentially damaging (the) nature reserve? Are we setting the precedent that, as long as we pledge to be careful, we can do infrastructure works in protected areas?"
Mr Subaraj Rajathurai, director of Strix Wildlife Consultancy, said: "This island used to be covered in rainforests; today we are down to 3 per cent. It has taken eons to evolve and the biodiversity is irreplaceable. Homes, however, can be cleared and rebuilt."
Mr Sangameswaran, 69, president of the Yew Lian Park Residents' Association, which has called for the CRL to go through the reserve instead of their 216-home private estate, said: "I think the public will be up in arms about the $2 billion. They will want the justification."
The LTA said the proposed tunnel beneath the CCNR will be about 40m deep and no surface structures will be built.
A second phase of the Environment Impact Assessment, to be completed this year, will study the environmental impact of the construction of both alignments.
Transport consultant Bruno Wildermuth, who was involved in the building of Singapore's first MRT line, said the $2 billion must be viewed in the context of the financial viability of the entire CRL and how many people it will serve.
Dr Park Byung Joon, adjunct associate professor at SIM University, said skirting around the nature reserve involves "social costs", including increasing end-to-end travel time to around four minutes more.
He said an extra $2 billion for 5km more of train tunnelling was "a bit expensive" and showed that there may be construction challenges. "The alignment is substantially curved and the costs to maintain the line may go up," he said.