Train operator SMRT said yesterday that it has identified one of two men seen smoking in one of its trains in a video which has appeared online.
The duo can be seen passing a cigarette between each other and taking turns to puff on it while a friend films them on a train travelling in the Bukit Batok area at night.
The clip was posted on Facebook last month.
Under the Rapid Transit Systems Act, anyone found smoking on railway premises can be fined up to $1,000 in court.
The offence can be settled out of court by paying up to $500.
In a statement, SMRT said: "This is indeed a brazen act. With the help of the authorities, we have identified one of the persons involved and have issued a Notification of Offence.
"SMRT takes a serious view of offences committed in our transport network and on board our trains and buses."
It added: "We hope that commuters will step forward to alert our station staff if they notice anything amiss in our stations, interchanges, trains and buses."