A train at SMRT's Bishan depot was vandalised, in the third such incident here in four years.
Police said they received a call early on Monday morning at 6.17am, requesting assistance "at a premise along Ang Mo Kio Avenue 1".
When the police arrived, a case of vandalism was reported, said a spokesman, who added that investigations are ongoing.
SMRT spokesman Alina Boey said on Saturday: "Vandalism was found on one of our trains at Bishan Depot on May 5. We have since made a police report and will assist the police in their investigations."
A spokesman for the Land Transport Authority (LTA) added: "SMRT discovered the graffiti on the exterior of a train at Bishan depot on the morning of May 5 and reported the incident to the police and the LTA. LTA takes a serious view of the incident and is currently working with the police and SMRT on the investigations."
SMRT has previously been fined $200,000 and $50,000 for two separate security breaches at its depots.
In May 2010, two vandals cut through the fence of SMRT's Changi depot and spray-painted graffiti on one side of a train. One of them, Swiss national Oliver Fricker, was given seven months' jail and three strokes of the cane, while his accomplice, Briton Lloyd Dane Alexander, remains at large.
In August 2011, a hole was cut in the fence at the Bishan Depot, and the words "Jet Setter's" were spray-painted on one of the trains.