Firefighters with the Ontario Fire Department responded to the 5400 block of Francis Street around 10 a.m.
According to the National Transportation Safety Board, the conductor was backing up when the train crashed into a combination vehicle.
No further details were provided, but NTSB said it's sending a team to Ontario to investigate.
"We are saddened by the loss of a Union Pacific employee who died on the job Wednesday in Ontario, California, and extend our deepest sympathies to his family and the employees who worked alongside him," read a statement from Union Pacific.
This is a developing story. This article will continue to be updated as more information becomes available.