On May 6, 2015, at 7:21 a.m. central daylight time, a BNSF Railway (BNSF) crude oil unit train derailed six cars (81 through 86) near Heimdal, North Dakota. The train, consisting of 3 locomotives, 2 buffer cars, and 107 loaded tank cars carrying crude oil, was operating at 45 mph when the cars derailed. The train separated after a broken wheel on the 81st car struck the leading edge of the highway rail grade crossing at milepost 149.01. A mark on the track structure at milepost 153.87 indicated that the broken wheel could not maintain its normal position on the rail at that point and the derailment sequence began. The momentum of the train pulled the 81st car and the following five cars off the track. Five of the derailed tank cars breached and released about 96,400 gallons of crude oil, which fueled a fire about 1 mile east of Heimdal. About 30 people were evacuated from Heimdal and the surrounding area due to the smoke plume that extended north. At the time of the accident, the sky was overcast and the temperature was 57°F. BNSF estimated damage at $5 million.