A Union Pacific Railroad freight train struck a big rig Tuesday in Montebello, causing no injuries but slightly delaying some Metrolink trains on the Riverside line, authorities said. The accident was reported about 6 a.m. at Vail Avenue, the Montebello Police Department reported. The big rig was hauling frozen food, police said. The circumstances of the crash were under investigation. Scott Johnson of Metrolink said slight delays were reported on the Riverside line, with passengers being bused between the Montebello/Commerce Metrolink station and downtown Union Station in downtown Los Angeles.