SMRT discovered graffiti on the exterior of a train at Bishan depot early this morning, before the train was put into service.
The incident was reported to the Police and the LTA.
Following earlier incidents of security breach, the Public Transport Operators (PTOs) have taken steps to enhance the security at the train depots.
These include increasing the number of security personnel and frequency of security patrols at the depot, as well as installation of concertina wires to reinforce certain stretches of the perimeter fencing.
Today's incident shows that security threats continue to evolve and agencies will continue to work to address security challenges faced.
LTA said earlier that it takes a serious view of this latest incident and is currently working with the Police and SMRT on the investigations of the 2014 incidents, to determine the cause of them.