NTSB determines cause of Union Pacific derailment in Tempe, Ariz.

Written by David C. Lester, Managing Editor
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The NTSB has released its completed investigation of a Union Pacific derailment.
Union Pacific

The National Transportation Safety Board released its completed investigation report on a Union Pacific derailment that occurred nearly two years ago. The derailment cased a hazardous material release and fire.

What Happened

On July 29, 2020, about 6:06 a.m. local time, a northbound Union Pacific Railroad (UP) freight train, MTUPX-29, derailed 12 railcars at the south end of the railroad bridge over Tempe Town Lake on UP’s Phoenix Subdivision in Tempe, Arizona.[1] At the time of the derailment, train MTUPX-29 was a mixed freight train consisting of 3 locomotives and 97 railcars. One of the derailed railcars struck the bridge structure, and part of the bridge collapsed, dropping railcars and bridge structure onto Rio Salado Parkway below and temporarily shutting down the road. Five of the derailed railcars were DOT-111 tank cars carrying various hazardous materials.[2] Two of these tank cars, both carrying UN1915 cyclohexanone, a flammable hazardous material, fell from the bridge during the​ ​​derailment; the rest remained in-line with the track. [3] One of the fallen DOT-111 tank cars released about 2,200 gallons of cyclohexanone, creating a pool of hazardous material below the damaged bridge. The cyclohexanone did not ignite, and no other tank cars were breached. Derailed lumber cars, which remained on the bridge, caught fire during the derailment. Some lumber fell onto and near the road, where it burned for several hours. (See figure 1.) There were no fatalities; one firefighter was treated for smoke inhalation at the scene. UP estimated the damage costs to be about $485,000 for railroad equipment, $435,000 for track and structures, and $10 million for the bridge.​

Fig. 1. Aerial photograph of derailment scene. (Union Pacific)

What the NTSB Found

​​The NTSB determined that the probable cause of the Union Pacific Railroad freight train derailment and subsequent bridge collapse on July 29, 2020, was a broken rail located on the ballast deck portion of the wooden trestle approach about 30 feet from the steel bridge. Contributing to the severity of the derailment was the absence of an inner guard rail preceding the steel bridge structure, which allowed the derailed equipment to move laterally into the bridge structure and cause its collapse.​


  1. (a) Visit ntsb.gov to find additional information in the public docket for this NTSB investigation (case number RRD20LR005). Use the CAROL Query​ to search safety recommendations and investigations. (b) All times in this report are local time unless otherwise noted. Because Arizona is one of two states that did not go on daylight saving time during the summer of the derailment, local time in Tempe aligns with Pacific Daylight Time. (c) The UP Timetable for the Phoenix Subdivision lists train movement as either eastward or westward, but for clarity, this report will use geographic direction for train movement and track references. Thus, as the train was traversing the Tempe Town Lake Bridge, this report describes it as traveling north.
  2. ​​​The rest of the 12 derailed railcars were mixed freight.
  3. Cyclohexanone is classified as a Class 3 flammable liquid, packing group III. The safety data sheet listed cyclohexanone by its trademark name Nadone® and described the material as a clear, colorless liquid with a “mint-like acetone-like” odor.

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