The replacement of MRT rail junctions requires more time now than before because of higher service frequency and competing works that are going on after revenue service.
At a briefing to explain why a number of stations along the East-West Line will close earlier from next Friday, operator SMRT said yesterday such replacement works could previously be completed after operating hours.
But that is no longer feasible.
SMRT Trains deputy director of permanent way projects Kelvin Tan said that, typically, engineers would lock up the switching rail at a junction - the part which moves to allow trains to switch to another direction - for the entire day to facilitate the replacement work.
Trains at a terminal station would then have to turn around at another nearby junction - something which takes a bit more time.
When trains were running at a lower frequency "four to five years ago", this was manageable.
But Mr Tan said locking up a junction today would cause a long queue of trains and unacceptable delays. He added that other projects in the network, such as resignalling and third-rail replacement, meant that there were shorter windows for the junction-replacement works.
Mr Tan said engineering hours - the time after the last train pulls into the depot and before the first train runs in the morning - have "always been insufficient".
"But because of all the other activities today, things have become even tighter," he said.
This is why SMRT is ending service between Lakeside and Joo Koon, as well as between Tanah Merah and Changi Airport, earlier at 11pm on April 14 and 21 (Lakeside-Joo Koon), and April 28 and May 5 (Tanah Merah-Changi Airport).
Shuttle buses will be provided at these stations instead.
There are some 170 junctions in the North-South/East-West line network. Because junction rails are exposed to greater wear and tear, they are replaced every three to five years, compared with 10 years or more for the rest of the tracks.
The work is more complex than replacing normal tracks because it involves dismantling sensors and drives attached to the rails.
Then, the rails have to be cut at three places. The rails come in a set of two pieces (18m each) and weigh about 1.2 tonnes.
These have to be aligned with precision and then welded. The welding of each point takes about 90 minutes.
Next, the welded points are ground to smoothen them. Sensors and drives are then re-installed. The tracks are then measured and tested.