Probably because Pinkerton had experience in policing and detective work. Pinkerton was the first police detective in Chicago, Cook County, Illinois. In 1850, formed the North-Western Police Agency, which later became Pinkerton & Co, and finally Pinkerton National Detective Agency. Pinkerton's agency solved a series of train robberies during the 1850s, and was through these that Pinkerton came into contact with George McClellan, who was the Chief Engineer and Vice President of the Illinois Central Railroad.