On June 20, 2006, about 2:25 a.m. Eastern Standard Time (EST), Norfolk Southern Railway Company (NS) eastbound freight train 166T519 consisting of four locomotives, 68 loads, 14 empties and 8,356 tons, derailed 21 cars at milepost (MP) 168.6A at Sweetwater, Tennessee (TN). Train 166T519 was operating on the main track of the NS Central Division, West End District and originated at Chattanooga, TN where the train received a class 1 brake test and inspection. The train crew consisted of a locomotive engineer and conductor. They reported for duty at Chattanooga at 8:30 p.m. on June 19, 2006, after receiving the prescribed rest. The train departed Chattanooga at 10:25 p.m. on June 19, 2006, and was 5,323 feet in length. Train 166T519 was traveling at a speed of 42 miles per hour (mph), 88 amps in no. 1 throttle, as recorded by the event recorder on the third locomotive (NS 8766) when an undesired train line induced emergency brake application occurred. Inspection of Train 166T519 by the conductor revealed the 14th through 34th head cars had derailed. The 21 derailed cars (17 loads and 4 empties) included five hazardous material loads with none leaking. As a precautionary measure, local emergency management officials evacuated a 12 mile radius affecting about 65 people. The method of operation in the accident area is Traffic Control and Track Warrant. The class of track is class 4 and is a single main track consisting of 132 lb. continuous welded rail (CWR) and wood ties. About 625 feet of track was destroyed. NS reported $768,727 in equipment damage and $35,000 track damage. At the time of the derailment, the weather was dark, the temperature was 70DGF. There were no injuries. The cause of the derailment was the coupler pin retainer bolt nut failed on the A(east) end of Car TTJX 80903 allowing the coupler to come out intact resulting in the derailment of TTJX 80903 and the following 20 cars.