The tornado started at 3:30 pm about 1.2 miles north of Poplar Grove in Boone County and ended at 3:48 pm about 3.2 miles north northeast of Harvard in McHenry County, Illinois.
At one point the tornado moved across the Chicago and Northwestern railroad line where it blew 12 railroad freight cars off the track. The train was moving at the time the tornado crossed the rail line hitting the train in the middle blowing the cars off one by one until it reached the heavier locomotive. As the engine stopped the remaining cars at the end of the train and still on the track continued to roll along until they slammed into the engine. This caused a few more cars to derail including a tank car containing hazardous materials that caused the evacuation of the town of Lawrence.
You can read the National Weather Service report here.