The FBI says it got the tip from a foreign government source Monday and passed it along to authorities.
The Universal City Metro Red Line station was the object of a potential threat that was to take place Tuesday.
More police and sheriff's deputies, K-9 bomb teams and undercover officers have been deployed along the transit system.
Authorities briefed reporters Monday because of the specificity of the threat and assured commuters that there would be increased patrols and the Metro lines would be safe.
"This could be real, it could be a hoax, but we must remain calm but vigilant," Los Angeles County Sheriff Jim McDonnell said, according to CNN affiliate KABC-TV.
Los Angeles FBI Field Office Assistant Director Deirdre Fike said the anonymous threat came from a man and particularly identified the Red Line that stops at Universal Studios in Los Angeles.
Los Angeles Mayor Eric Garcetti took the train Tuesday from Universal City and reassured people of the line's safety.
"One of the lessons from today is all of us can be the eyes and ears on this system and throughout the city," Garcetti said.
There was a visible presence of security from local law enforcement and the US Department of Homeland Security at Union Station, the main hub for the Metro subway, CNN's Casey Wian reported.
There were at least double the number of law enforcement personnel one usually sees during the morning commute on the Metro Red Line, Wian said.
"Passenger volume also seemed lighter than normal, and I noticed several passengers as they entered subway cars looking both ways and carefully scrutinizing the car before choosing a seat," Wian said.
CNN's David Daniel rode southbound Tuesday morning on the Red Line. "I boarded at North Hollywood at 8 a.m., and while there was a large and obvious security presence both outside and inside the station, the ridership level seemed fairly normal," Daniel said.