Police are hunting a gun-wielding mugger who they say tried to steal cash and cards from a teenage passenger on a train.
The man is said to have pulled out the weapon to threaten a 19-year-old aboard a local service in north London last Friday.
After unsuccessfully demanding his victim hand over money and his bank card, the robber then fled the train.
Detective Sergeant Dean Percival said: “This is an incredibly concerning incident and we are doing everything we can to trace the individual involved and remove this potential weapon from the streets.”
"The man demanded cash and the victim's bank card before reaching into his bag and pulling out a silver revolver handgun,” Det Sgt Percival said.
"It is not yet known whether this was a viable handgun or an imitation firearm.”
As the train arrived at Brimsdown, the mugger calmly walked off the train while the victim ran through the carriages to alert the driver.
Police are urging anyone who recognises the man in the CCTV images to contact them as quickly as possible.
Last week a “devious and dangerous” man was jailed for at least 28 years after viciously stabbing a fellow train passenger to death on board another London-bound train.
Darren Pencille launched the unprovoked attack after a row with his victim, 51-year-old father and IT consultant Lee Pomeroy, over who was blocking the aisle.