A Thameslink train driver caused two terror alerts when he mistook a didgeridoo for a grenade launcher.
Armed police arrived to find a didgeridoo player
He reported seeing a man apparently taking aim with a weapon at Herne Hill station in south London.
Armed police arrived but the man had gone. Other passengers told officers he had been carrying a didgeridoo.
But no-one told the driver and he again reported seeing an armed man at Loughborough Junction. Armed officers arrived to find a didgeridoo player.
Inspector Ian Hampton, from British Transport Police, said they along with armed response units from the Met were alerted on Saturday.
The driver said he had seen a man in "camouflage gear with a grenade launcher and taking aim".
"At the first incident at Herne Hill we arrived but the man in question had gone," he said.
"Other passengers said the man fitting the driver's description of the suspect had been there but only had a didgeridoo.
"Apparently he had been playing it in the town centre.
"Then later that day, the same driver reported seeing the man taking aim at Loughborough Junction. This time we found the man with the didgeridoo."
He added: "The driver was quite right to inform us about what he genuinely thought was a terrorist threat on both occasions.
"We would not want to stop anyone reporting what they feel is a genuine risk to people's security."