Mechanical problems forced the Union Pacific train to stop on Metrolink's Riverside Line east of the Industry station, resulting in the cancellation of two commuter trains -- the 402 and 411.
Metrolink, which serves six Southern California counties, shares the track with Union Pacific, the nation's largest freight railroad.
Jeff Lustgarten, a Metrolink spokesman, said buses are being provided to stranded passengers.
He added that Union Pacific told Metrolink that the problem should be solved by 4 p.m. If not, he said, the blocked track could severely impact rush hour service on the line.