Tempe firefighters, paramedics and police rushed to a rail bridge over Tempe Town Lake, following the 6:15 a.m. MT derailment which sent smoke plumes visible for miles.
The bridge where the accident happened was inspected on July 9 according to Union Pacific.
"The south side of the bridge collapsed and rail cars fell into an empty park below," Union Pacific said in a statement. "Three tank cars were on the ground under the bridge. Two contained cyclohexanone; one contained a rubber material. None are reported leaking, and no tank cars were involved in the fire."
The National Institutes of Health describes cyclohexanone as a flammable liquid, used primarily in the the production of nylon, that is harmful if inhaled.
The NTSB is investigating.
There were no immediate reports of casualties from the derailment and fire, Tempe Mayor Corey Woods and Police Chief Sylvia Moir both said.
"The No. 1 thing when someone like me gets out of bed and they see what happened with this train derailment and the collapse of the bridge, that is the main thing we look for is: 'Is everyone OK, did anyone get hurt?'" Mayor Woods told NBC affiliate KPNX.
"And I'm pleased to report that so far we have definitely not gotten any reports of any fatalities as a result of this incident."
Lumber being carried on the Union Pacific line was ablaze Wednesday, and hazardous materials crews were dispatched to the scene, officials said.
With burning cars on a bridge over water, firefighters struggled to get close to the flames.
"Being over the water definitely adds some ... challenge for us," Tempe Assistant Fire Chief Andrea Glass told KPNX. "Unfortunately there are some cars that will be lost because we just can't get to them, they will burn."
The accident forced closure of Loop 202, a major thoroughfare of the metropolitan Phoenix area, in both directions near the fire, the Arizona Department of Transportation said.