British Transport Police (BTP) officers were called to remove the teen from a Great Northern train service to the station, one of the busiest in London, on Saturday afternoon.
A sword “concealed down his trouser leg” was seized from him by BTP officers based at King’s Cross said.
He was detained and is being held in custody.
BTP tweeted a picture of the terrifying large blade, which appears to be a replica sword with an gem-encrusted handle.
British Transport Police tweeted: “Officers were called to Kings Cross earlier and removed a 15 year old male off a @GNRailUK service. This was concealed down his trouser leg. Arrested & currently being dealt with in custody.”
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