A 52-year-old man from Watford has been arrested after a video was released of a Tube passenger using "racist language" on board a train in London.
British Transport Police said the man was arrested on suspicion of a racially aggravated public order offence.
"An investigation was launched after we were made aware of a YouTube clip of a man using racist language on board a Bakerloo line train," police said.
The man was bailed pending further inquiries.
The incident is believed to have happened on Tuesday at about 21:00 BST.
He was bailed to return to a police station on 30 October.
British Transport Police are appealing for witnesses.