The first trains start to pull into the Sengkang Depot, a protected high-security area, after 10pm.
From then, it is a race against time.
All maintenance work has to be completed by 4 o'clock the next morning so that the trains can roll out to service the peak-hour crowd.
As soon as a train reaches the depot, it is checked for faults and repaired, if needed.
If no defects are found, a team of cleaners moves in for primping and polishing.
The floors are swept and the seats and windows wiped.
The cleaners are also on the lookout for any suspicious items, graffiti or vandalism.
After this, train functions such as the announcement system, doors, air-conditioning and lights are checked.
It takes about 30 minutes to process each train, assuming it does not need repairs.
Once every two months, each train gets a thorough scrub.
Built at a cost of $4.6 billion, the decade-old North East Line (NEL) is one of the world's first fully automated underground rail systems.
More than 20 trains ply the NEL daily, which has 16 stations and is 20-km long.
The maintenance regime of the track and tunnel is carefully planned throughout the year. They are visually inspected twice a week - all within three hours when the trains are in the depot.
Combing the dark tunnels in their protective gear, workers also carry out regular preventive and corrective works. Every fortnight, a specialised Multi-Function Vehicle equipped with ultrasonic rail flaw detection system inspects the track and tunnel for displacement or any components that may pose as obstacles to the trains.
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