The man who was killed was an employee for the Union Pacific Railroad and was part of a crew working on freight tracks.
"The pickup truck was in pieces," Metrolink spokeswoman Angie Starr said.
There were 142 people on the train. Two passengers were taken to the hospital with minor injuries.
Officials said it is unclear if the maintenance truck was crossing the tracks, or if it was being driven down the tracks using metal train wheels can be hydraulically lowered by many Union Pacific maintenance trucks.