The e-mail came like a bolt out of the blue - Austrian builder Alpine Bau, the main contractor for a three-station stretch on the Downtown Line (DTL), had gone bust.

Overnight, work along 2.1km of the DTL ground to a halt.

It was an unprecedented situation, even for Land Transport Authority (LTA) project directors Ng Kee Nam and Tan Kian Thong, who have more than 50 years of rail engineering experience between them.

"This was quite unexpected. Progress was good and we had no signs," said Mr Ng, deputy group director of rail (civil), adding that when contractors go bust, there were often warning signs - such as fewer workers turning up at the worksite or suppliers showing up to cart away equipment.

But with Alpine Bau, there was none of that. It meant the Downtown Line 2 (DTL2), which would link the north-western corridor to the Marina downtown area, would not be able to open as planned by this year.

The company was responsible for three stations - King Albert Park, Sixth Avenue and Tan Kah Kee - that were smack in the middle of the 16.6km 12-station DTL2.

The DTL1, which spans six stations in the downtown area, first opened in 2013.

Optimistic estimates added six months to the deadline because of the company's insolvency.

Two years on, the DTL2 is opening as planned on Dec 27 - a feat both Mr Ng and Mr Tan credited to the ingenuity, team spirit and derring-do of workers and engineers involved.

They pulled additional shifts, sometimes working round the clock, and took calculated risks to get the job done, said Mr Ng.

One of the first steps LTA had to take was to call for a tender for a new contractor and this they did in record time - six weeks, with the contractor appointed two weeks later.

Normally, a tender would require half a year for processing and evaluation, said Mr Ng.

His engineers also had to secure the site and document the progress of work before a new tender could be called.

"This period of time was the most stressful, because we knew the faster we could award (the tender), the lesser the impact on DTL2," noted Mr Ng, adding that he suffered sleepless nights and worked almost every weekend.

At that point, work on the stations was about 60 per cent complete, while tunnelling had barely begun.

Alpine Bau had left four tunnel boring machines (TBMs) under Bukit Timah Road, which were fast becoming a safety risk, said Mr Ng.

"There were concerns... if we left the TBMs too long in the ground, later on we might not be able to restart them as they might get jammed."

So a caretaking contract to continue tunnelling was issued to Australian contractor McConnell Dowell, which was responsible for work just north of the affected area.

And when two new contractors eventually took over work on the three stations, it was all hands on deck.

Manpower was boosted by 25 per cent and workers worked overtime, with an additional graveyard shift that worked through the night.

Every effort was made to save time, and calculated risks were taken so work could be done concurrently throughout the site.

This was highly unusual as work was usually performed in stages.

For example, civil engineering works in a room would be completed before painting or wiring starts.

"A contractor could be doing up one part of a room, while civil engineers could be still cutting something at another part," said Mr Ng, adding that although risks were taken, safety was always a priority.

Precautions were taken, such as using fire blankets while welding work was carried out, said Mr Ng.

The combined effort clawed back time, but it was only in June, when work was about 95 per cent done, that engineers were confident they could stick to schedule.

Their feat so surprised ministry officials that both Mr Ng and Mr Tan - who are now working on the Thomson-East Coast Line - were asked if completion dates could be advanced at other projects.

Said Mr Tan: "We did it then, but it was not ideal in many aspects."

Meanwhile, commuters are just relieved they will be able to ride the trains as scheduled.

Retiree Madam Wang Yi Poh, 75, will be able to hop on the train to visit her three grandsons in Bukit Panjang.

"With the MRT it will be very easy to go now. I don't have to rely on my son to pick me up," said Madam Wang, who lives near the Beauty World Station.