Two men were detained and questioned by police at the Bishan MRT depot after they were seen taking photographs of the secured facility yesterday morning.
The Straits Times understands that the men boarded a shuttle bus which ferries personnel into the depot. But at the security gantry, they were found not to have any authorised passes for entry.
The men, believed to be foreigners, wanted to take photographs of the place, and started using their cameras at the gate.
An SMRT spokesman said: "The vigilance of security personnel at Bishan Depot resulted in them approaching two individuals who were seen taking pictures of the train depot.
"The depot is gazetted as a protected place. Our security personnel detained the two suspects and handed them over to the police."
When The Straits Times arrived at the scene at about 11.45am, the two men were seen surrounded by at least 10 police and security personnel. Three patrol cars were also at the scene.
The two men were questioned for more than half an hour just before the security gate. They were then led into the security guardhouse before being released.
The police said they were alerted by security officers at about 10am.
"The police interviewed and conducted checks on the two men. Nothing incriminating was found on them and no arrest was made," the police said in a statement.
Bishan Depot has been broken into by vandals in the past.
Out of four vandalism cases involving train depots between 2010 and 2014, two breaches were at Bishan Depot.
Since then, security at all rail facilities has been tightened.
"The safety and security of our transport network is a key priority for us," the SMRT spokesman said.