Work on Canberra station on the North-South Line (NSL) started yesterday, as Transport Minister Khaw Boon Wan stressed that safety will be a top priority in building the station - only the second after Dover to be built over an MRT line already in operation.

Canberra station, which is between Sembawang and Yishun stations, is expected to be ready in 2019.

"Building a new station on a functioning line has its challenges," Mr Khaw wrote on Facebook, adding that he has "stressed to the construction team to place safety as their top priority".

Mr Khaw's emphasis on safety comes after two SMRT workers were hit and killed by an oncoming train near Pasir Ris MRT last Tuesday. SMRT admitted a day later that a safety lapse led to the deaths of the men, aged 26 and 24.

In a press release yesterday, the Land Transport Authority (LTA) noted that stringent safety measures will be put in place.

"We have enhanced the work processes used previously for the construction of Dover station," said LTA. Dover station, on the East-West line, opened in 2001.

For Canberra station, critical works next to the existing tracks will be confined to the three to four hours when the trains are not running, said LTA.

When works are carried out near the "live" tracks, LTA and the contractor will coordinate with SMRT, the NSL operator, over access to the work area, and impose stringent control on the works being done, it added.

A project safety review process for the design and construction phases has been implemented, along with detailed risk assessments, LTA said.

A protective enclosure of nearly 220m - the length of the station - will be built around the tracks for safety. This is so that light construction works, such as the outfitting of the skin of the roof, can be done even as trains are running.

Working around an operational MRT line presents a unique set of engineering challenges, especially as the hours to do critical works are limited, said LTA's director of rail expansion Chuah Han Leong.

"You are building all around the MRT line - building under because the concourse is there, building next to it as the platforms are on both sides, and a roof on top. You are wrapping around the viaduct, basically," he said.

A station mock-up will be built off-site so a trial installation of the columns and roof truss can be done, to determine the safest and most effective construction method, Mr Chuah added.

The mock-up will be a skeletal version and will comprise two sets of columns and roof trusses.

During construction, the roof trusses, which span more than 28m, will have to be lifted vertically by cranes and joined to columns of about 17m in height. "You can imagine the precision required," Mr Chuah said.

As part of the works, a new 72m- long rail crossover track will be constructed north of Canberra station to connect the two existing tracks. "This allows trains to move from one track to the other, providing better operational resilience," LTA said.

The contract to build the station was awarded last April to China State Construction Engineering (Singapore) for $90 million.

When completed, Canberra station will serve residents of nearby estates like Sembawang Springs as well as upcoming residential developments.