Security has been boosted across the Los Angeles metro following a tip from overseas about a possible bomb attack against one of the stations in the network, authorities have said.
"This threat is imminent … it is very specific," Los Angeles police chief Charlie Beck told a news conference.
"But the credibility still needs to be vetted."
Law enforcement officials said the threat had been relayed to the FBI by a law enforcement agency in another country, where the threat had originated. Officials did not identify the country.
The male caller, speaking in English, had warned authorities in that country of an attack against a Red Line station across the street from the Universal Studios theme park today, Mr Beck said.
The caller did not threaten to carry out the attack but said he was alerting law enforcement.
The Red Line runs between downtown Los Angeles and northern neighbourhoods, including Hollywood and north Hollywood, and carries about 145,000 passengers a day as part of the city's larger transit system.
Chief Beck and Los Angeles County Sheriff Jim McDonnell said security would be stepped up at the Universal City/Studio City station and elsewhere along the city's rail lines.