A spike in construction cost has contributed to the MRT Downtown Line bill rising by more than 70 per cent from an original estimate of $12 billion to $20.7 billion.
Half of this $8.7 billion increase was attributed to a sharp rise in construction cost, with the other half linked to a number of changes to the project.
This was revealed to The Straits Times by the Land Transport Authority (LTA), which also said it is taking steps - like inviting more firms to bid for projects - to rein in prices.
The Downtown Line will link the north-western and eastern parts of Singapore to the Marina downtown, with the first stage due to open next year, and two other stages in 2015 and 2017.
When the line was announced in 2008, the LTA said that it would cost $12 billion. The latest price tag appeared in this year's Budget statement.
The changes adding to costs include expanding the capacity of the depot in Gali Batu near Woodlands, more stringent safety and regulatory requirements, and more connections to nearby developments to make the line more accessible.
For example, Downtown Line Stage 3's Tampines and Expo stations will each have an additional entrance.
The project is also slightly longer than when it was first announced in 2008. It is 42km long, with 34 stations - from the original 40km and 33 stations.
But construction cost remains the single biggest factor for the cost spiral. LTA deputy chief executive Chua Chong Kheng said that the increase in construction cost from late 2007 to mid-2008 was "quite significant".
He attributed this partly to the deluge of infrastructural projects locally as well as overseas - especially in China - which caused a shortage of building materials, equipment and manpower.
Downtown Line accident site Click on thumbnail to view. Story continues after photos. (Photos: AFP, BH, SCDF, ST, TNP, Wanbao)