The crash around noon Monday closed Interstate 8 in both directions for about an hour near Dateland, about halfway between Gila Bend and Yuma.
A Yuma County Sheriff's spokesman says the train was hauling automobiles and the tanker was trying to cross the tracks at an uncontrolled crossing.
The train struck the trailer portion of the fuel truck, causing the cargo of about 4,500 gallons to catch fire.
Authorities say the two front engines of the train were heavily damaged.
They say the cause of the incident is under investigation and the Union Pacific employee was transported to a Yuma hospital for treatment.
Three Marines in the tanker truck escaped injury.
Copyright 2015 The Associated Press modified.