Chicago was well launched in the meat packing business by the 1830s, but challenges and opportunities began to pop up. The Illinois and Michigan Canal and the first railroad opened in 1848. In 1852 Chicago was connected by railroad to the East.
Meat packers no longer had to depend on local farmers driving livestock in from the country. It also meant that live cows could be shipped through Chicago directly to the East. If easterners lost their taste for salt pork in favor of fresh meat, the industry would be in trouble.
When the Civil War started in 1861, nine railroads terminated in Chicago, including the strategic Illinois Central which headed directly south to the battlefields. The Union Army had up to a million soldiers to be fed, creating a huge demand for salt pork and beef. Live animals were also sent to the battlefronts to be slaughtered and eaten.
The Chicago meatpacking and shipping business exploded. There were slaughterhouses up and down the North and South Branches of the Chicago River. The 1863 U.S. Department of Agriculture Report has an excellent description of one such Chicago packinghouse. Each of the railroads had one or more stockyards (sort of a passenger depot for livestock) associated with it.
It was difficult to move stock from one railroad to another or to the slaughterhouses. The City Council was getting worried over water and air pollution. The slaughterhouses were upriver, and with the right wind the stench carried.