from what i know, they have pple in plain clothes, probably from isd, going around the stations to look out for mysterious pple and objects.
I'm pleased to know that if & when SG economy nose dives & crashes, ISD will still be hiring people for this kind of work, why don't we all form a queue & specifically ask for this job, sounds like a good job to me, everyday take mrt for free & knowing that you are helping out the economy.