Two railroad tank cars, each filled with 24,000 gallons of deadly anhydrous ammonia, were involved in a spectacular derailment in Benson, Arizona, at approximately 0930 hours on a Sunday morning. A precautionary evacuation was conducted, but no anhydrous ammonia was discharged, and the incident was eventually resolved without deaths or injuries.

The Benson Fire Department was on the scene within minutes of the 19-car derailment, which occurred in a light commercial and residential area of town. A string of cars not connected to a locomotive at the time apparently started running downhill from a nearby siding where they had been stored. The cause of the incident is still under investigation. The cars, traveling at an estimated 40 miles per hour, hit an automatic derailment device, a termination point on the tracks.

The majority of the tank cars were empty, and the violent derailment created a huge pile, six cars high. Only a loaded fertilizer car, filled with a nonhazardous material, caused any additional concern, and it was safely uprighted after it lost about 150 gallons of product.

The derailment posed the potential for extensive mass casualties because of the toxic nature of the anhydrous ammonia, which had caused a fatality earlier that year at an agricultural supply facility west of Phoenix. For domestic transportation, the U.S. Department of Transportation classifies anhydrous ammonia as a nonflammable gas. However, in international transportation, it is classified as a poison gas, since it meets this category’s defining criteria with an immediately dangerous to life and health (IDLH) value of 500 parts per million.

The Benson Fire Department requested assistance through the state Division of Emergency Management, which is charged with coordinating the hazardous-materials response activities of state agencies within Arizona. The Arizona Department of Public Safety (DPS), which has a statewide hazardous-materials team, dispatched two technicians and a specialist to the scene. Under the provisions of the integrated state plan, the DPS provides state-on-scene coordinators (SOSC) during the “fires-and-explosions” phase of an incident. Once the scene is stabilized, the most appropriate state agency assumes the SOSC duties.

(1) The 19-car derailment occurred on a Sunday morning after the string of cars, not connected to a locomotive, started running downhill from a siding where they had been standing. (Photos by author.)


(2) The downwind area at the time of the derailment was sparsely settled desert terrain, but the incident was immediately across the street from a residential and light commercial area of the small southern Arizona town of Benson.



At the request of the Benson fire chief, the DPS assumed command of the scene, using Benson Fire to provide vapor suppression, first aid, and stand-by rescuers and the police department to handle scene security, traffic control, and any additional evacuations that might be needed.

Although the siding with the derailment was operated by another local railroad, the Union Pacific/Southern Pacific Railroad (UP/SP) responded to the scene, since its mainline southern Arizona tracks were several feet away. UP/SP supervisors agreed to handle the actual wreckage clearance and immediately dispatched work crews and contractor equipment to the scene. The railroad summoned a member of its pipeline subsidiary, who advised command that an eight-inch gasoline pipeline paralleled the railroad tracks in this area; however, it was on the opposite side of the track from the derailment and posed no hazard.

Railroad and DPS hazardous-materials specialists carefully examined both derailed anhydrous ammonia tank cars. Their shelf couplers had remained coupled, even though one car was 90° over on its side. One of the jacketed tank cars received significant damage, but the residual structural integrity appeared sound. To rerail both cars, they would have to be separated. Given the extreme forces on both couplers, the only way to separate them was with a cutting torch.

The railroad welder, assigned the separation job, had previous experience cutting coupler drawbars. The fastest and safest method was to use “burn bar” instead of a conventional welder’s cutting torch. Burn bar is a special combination alloy of magnesium and other hot-burning metals formed into a hollow six-foot-long bar, through which pure oxygen is injected. Once ignited, and continually fed by the oxygen, the burn bar generates intense heat for the heavy work of cutting the thick coupler drawbar.

(3) Most of the derailed tank cars contained only residue of non-hazardous liquid fertilizer, except for one loaded fertilizer car and two anhydrous ammonia-laden tank cars.


(4) The derailed anhydrous ammonia tank car, on its side, did not show any signs of major damage, so it was carefully picked up and uprighted by two “side-boom” tractors.


Once the plan was formulated and agreed to, the fire department, using its aerial ladder with a 1,000-gallon-per-minute fog nozzle, provided for vapor suppression. If a leak developed from either of the anhydrous cars during the operation, the water fog would provide additional time for evacuation. Anhydrous ammonia vapors have a strong affinity for water and can be washed out of the air with a fog stream.

The fire department also provided a “fire-watch” crew to assist the DPS hazardous-materials specialist on the far side of the derailed cars during the burn-bar operation. Given the combustible nature of creosote-soaked railroad ties and the possible presence of other combustible materials in the vicinity, a fire watch crew is an added precautionary measure whenever cutting or welding is done at derailments. The actual burn-bar operation took less than five minutes, and heavy contractor equipment then uprighted both anhydrous ammonia cars.

The righted cars were placed off to the side of the derailment area until the track could be rebuilt and the cars rerailed. Close visual inspection of the car that had been 90° degrees over on its side revealed no hidden damage on the down side; therefore, before sunrise the following day, the scene was declared stable. Under Arizona procedures, the SOSC duties then passed to the Department of Environmental Quality, given the potential that still remained for environmental pollution from the damaged liquid fertilizer cars. Finally, more than 24 hours later, the track was successfully rebuilt, the anhydrous ammonia tank cars were rerailed, and the incident was declared over.


  • The local fire department, recognizing the experience and capabilities of a state agency, turned incident command over to it, and a unified command operation was successfully conducted.
  • Early, rapid notification of the UP/SP railroad saved hours in the wreckage-clearing operation.
  • Recognizing the skills and capabilities of the railroad and its contractors allowed the operation to proceed efficiently and without friction.
  • Consensus decision making and extensive safety briefings can result in injury-free operations even at complex hazardous-materials accidents.

STEPHEN L. HERMANN is hazardous materials coordinator and specialist for the Arizona Department of Public Safety and Arizona’s senior state-on-scene coordinator for hazardous materials emergency response. He is past national chairman of COHMED, the national organization of state and local hazardous materials enforcement officers, and past chairman of the Commercial Vehicle Safety Alliance Hazardous Materials Committee. He has a bachelor of science degree in explosives technology and is a graduate of the U.S. Naval School Explosive Ordnance Disposal, United States Army Command and General Staff College, U.S. Army War College, and state Division of Emergency Services Hazardous Materials Technician course.

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