SMRT has yet to arrive at a firm conclusion on its latest vandalism case, but an early assessment suggests that disgruntled workers or vendors could be behind it.
It also appears that the vandals had more time to do their mischief at Bishan Depot than in May, when another train was targeted.
A photo of the vandalised train obtained by The Straits Times showed graffiti that was more elaborate this time around, although the graffiti in May's case also stretched across two sets of train doors.
There have been two other reported cases of defaced trains here. In 2011, a hole was cut in the Bishan depot fence and the words "Jet Setter's" spray-painted on a train. And in 2010, two vandals cut through SMRT's Changi depot fence and spray- painted graffiti on a train.
One of the vandals, Swiss national Oliver Fricker, was caught and jailed for seven months. Fricker, who also received three strokes of the cane, remains the only SMRT vandal nabbed so far.
The latest two incidents took place despite SMRT having raised security by installing motion sensors along its perimeter fencing, more cameras and more lights, and deploying guards from third-party vendors.
The Land Transport Authority said security threats continue to evolve, and agencies will continue to work to address security challenges faced.
A source with intimate knowledge of SMRT's security measures said of the latest case: "It is most likely an inside job. Whoever did it is very daring, very foolish or desperate, because they will be jailed and caned if they are caught."
But as the Bishan depot perimeter is 6.5km long, intruders cannot be entirely ruled out at this stage.
"It may take some time for them to examine it closely to determine if there had been any breach," he said.
SMRT said it was too early to conclude that the latest case is the work of disgruntled staff. Group spokesman Patrick Nathan said an internal survey last year found that 82 per cent of employees were "sustainably engaged". And about 90 per cent said they were "proud" to work at SMRT.
Dr Joel Yang, who heads the master of counselling programme at SIM University, said vandals have various motivations, including anger, frustration and vengeance.
"These theoretical overviews suggest that vandalism in general is motivated by negative feelings," he said. "However, other studies have also shown that vandalism can be motivated by curiosity, playfulness and pleasure."