A bomb squad gave the all-clear after investigating a suspicious package on a freight train in Sylmar on Wednesday.
The device was found just after 2 p.m. on the northbound train near San Fernando Road and Roxford Street, officials with the Los Angeles Police Department said.
A conductor, who said he saw a man who may have jumped off the train, found a yellow tube that resembled a pipe bomb attached to one of the sides of the train cars, said Sgt. Wes Albin of the LAPD's Mission Division.
It was not immediately clear how many people were on board the train.
No injuries were reported.
Other trains along the Antelope Valley Metrolink line are affected and buses are being requested to transport passengers, according to tweets from Metrolink.