A middle-aged man allegedly pushed to his death under a Tube train was wearing women's clothes, sources have said.
He died after he was struck by a train at King's Cross Underground station at 6.30pm on Monday, British Transport Police (BTP) said.
His companion was arrested at the scene on suspicion of murder. She had to be restrained by a fellow passenger until police arrived, BTP said.
Det Supt Ashley Croft said: "Early indications from witnesses are that the victim and the suspect knew each other.
"We are also in the process of viewing CCTV from the station and interviewing several witnesses who came forward last night.
"It's really important that we build the clearest possible picture of what took place on the platform and I ask anyone who may have seen the incident to contact us.
"I would like to thank passengers for their patience and understanding, as well as all of the London Underground staff who assisted the emergency services in dealing with what happened in a professional and dignified manner.
"Incidents like this are extremely rare on the Tube."
The station was evacuated and the Piccadilly Line was temporarily closed while police recovered the body.
The woman being questioned is a 34-year-old from Cricklewood, north-west London, police said.