Video images showed huge flames and thick black smoke rising into the air and three train cars in a park next to Tempe Town Lake.
None of the train's crew members were hurt but there was a report of someone suffering from smoke inhalation, said Tim McMahan, a spokesman for Union Pacific Railroad. He said he did not know how many crew members were aboard when the train derailed.
Fire officials in the suburb of Tempe said the derailment happened at about 6 a.m. and that about 90 firefighters were at the scene.
Between eight and 10 of the train's cars caught fire, McMahan said. Several carried lumber and the three that fell to the ground beneath the bridge were tank cars.
Two were carrying cyclohexanone, a pale and oily liquid that is toxic and flammable. McMahan said a third tank car was carrying a "rubber material" but did not provide further details.
McMahan said the tank cars were not involved in the ensuing blaze and it appeared none of them had any leaks.
Investigators with the Federal Railroad Administration were dispatched to start a preliminary investigation, the agency said in a statement.
The bridge had its annual inspection earlier this month, McMahan said.
Fire witness Camille Kimball told AZFamily.com she was riding her bike in the park below the bridge minutes before the derailment.
"I turned around to look and got the fright of my life," Kimball said. "Now there's fire pouring into the lake from the middle of the bridge. ... It looks like a scene from hell, truly."
The 2.5-mile (4-kilometer) Tempe Town Lake is is a popular recreation spot for jogging, cycling and boating and close to Arizona State University.