A teenager pleaded guilty to possession of an imitation firearm at Wimbledon station in court.
Philippe Valapinee, of Norbury, Croydon, was arrested at Wimbledon Station on Sunday, 6 April following reports of a firearms incident.
Armed officers were called to the station at 4.27pm on Sunday, April 6 and arrested Valapinee and another 18-year-old man.
There are no reports shots were fired or anyone was injured.
The second man was later released with no further action.
Valapinee admitted the offence at Wimbledon Magistrates Court yesterday.
He was bailed, to return for sentencing on April 23.
Police are hunting for a third man who was also reported to be at the scene.