The recent vandalism of an SMRT train that resulted in the arrest of two German men is worrying several quarters in the Government. And rightly so.
Not only was it the fourth such case in four years but, like the first three, it also involved trains that were parked in highly protected depots.
In fact, after the first two incidents in 2010 and 2011, SMRT installed a sophisticated intrusion-detection system, and roped in security contractors to patrol its depots.
Yet, two more incidents took place - in May and earlier this month. The latter was alleged to have been the work of two 21-year-old Germans, first-time visitors who had entered the country only a couple of days before the incident.
Member of Parliament Lim Biow Chuan, who is on the Government Parliamentary Committee for Transport, said the repeated security breaches were worrying.
"Today we have young people doing vandalism, tomorrow it may be someone else with a more dangerous intent," he said, echoing the sentiment of other parliamentarians.
As far removed as vandalism seems to be from sinister acts such as terrorism, the fear is not unwarranted.
Images of mangled steel and bodies in the aftermath of bomb attacks on rail lines in Moscow, Mumbai, London and Madrid between 2004 and 2010 are still vivid in people's minds.
The Madrid incident - where 10 coordinated explosions killed 191 and injured more than 1,800 commuters on March 11, 2004 - is especially chilling. Several of the bombs that went off were planted on trains while they were in stations.
According to the US National Transportation Security Center of Excellence's 2012 annual report, attacks on transit systems have been "extensive".
From Sept 11, 2001 to 2011, the centre noted that there had been close to 2,000 attacks resulting in more than 3,900 deaths and more than 14,000 injuries.
These attacks were also far more lethal than bombs onboard aircraft. On average, each transit attack resulted in 3.3 deaths - compared with two for aviation attacks.
The stark difference may be attributable to the nature of mass transit. A packed 10-car subway train can carry more than 2,000 people during rush hour, compared with about 850 that an Airbus A-380 carries.
While one may argue that there is practically no chance of survival in an airborne attack, the devastation of bombs going off on a train at a crowded platform - or in tunnels, where explosive forces are amplified - cannot be underestimated. The porous nature of a transit system also makes security far more daunting. In Singapore, more than two million train rides take place a day, with commuters using close to 300 entrances and exits across 100 or so stations.
And as our rail network doubles in length by 2030 to 360km, so too would ridership.
The stringent screenings air travellers are subjected to cannot be applied to transit systems, as they would render mass rapid transit far less rapid and hence inefficient.
It is perhaps for that reason that terrorists are targeting them. As observers have pointed out, tightened security at airports may also have made transit networks more viable targets.
Security expert Brian Michael Jenkins noted in a blog for American think-tank Rand earlier this year that terrorists having moved towards softer targets "can be interpreted as an indirect indicator that security works". "Criminals exploit the absence of security," Mr Jenkins wrote. "Increasing security drives them away."
That does not explain SMRT's four cases of intrusion and vandalism, despite its slew of security measures.
Local security experts were at a loss when asked to comment on the cases. Did the first incident in 2010, which attracted international media attention not so much for the crime but the punishment, throw down the gauntlet to the community of vandals?
No one knows. How the two Germans were able to enter a highly secured depot without breaking and entering also remains a mystery. As do the exact circumstances that led to their arrests.
For SMRT and the Government, the incident is a source of embarrassment. What is harder to swallow, though, is how it once again demonstrated the vulnerability of the transit system.
That Singapore also had a spate of other very public security breaches dating back to the high-profile escape of Mas Selamat Kastari in 2008 holds no comfort.
If these non-professionals - including a 28-year-old Malaysian teacher who drove past the Woodlands Checkpoint and into the premises of the Ministry of Foreign Affairs - could breach security measures with seemingly so little effort, what chance do we have against highly trained and motivated terrorists?
That may not be an entirely fair assessment. While it is easy to notice lapses, it is harder to quantify the overall effectiveness of security systems.
Mr Jenkins noted: "It is difficult to count things that don't occur. Many criticise security as being 'just for show'. However, illusion is an important component of security."
Metros in many major cities face the scourge of vandals. In Melbourne, an average of 35 trains are defaced each month by vandals, who sometimes damage brakes and signalling systems.
Even metros in orderly Japan are not spared. In recent years, vandals have targeted trains in eight cities across the country.
Singapore's four cases over four years thus do not appear to be alarming. Still, it would be foolhardy not to deal seriously with every security breach that comes to light.