Manufacturing Core
As mentioned perviously, Pittsburgh was a part of the Eastern manufacturing core in the U.S. Its location near three major rivers for access to other markets and transportation, and a supply of bituminous coal made in to a central player in the Steel Rail- Epoch from 1870 -1920. During this time steel began to replace the iron used in making railroads. The development of the city of Pittsburgh began through the United States Steel Corporation and became a steel producing center. Plants were created near Pittsburgh to take advantage of the low coast water transportation and the steel being created there. As the steel industry grew so did the city and an urban center was thus formed. AndrewCarnegie can be attributed to the reason Pittsburgh became a major steel town. In 1875 he opened the largest steel plant, the Edgar Thompson, in the Pittsburg area. And in 1899 he created the Carnegie Steel Company which soon became a major leader in the Steel and Railroad industry.

Sources:
http://pittsburgh.about.com/cs/famous/p/andrew_carnegie.htm
http://steelcityartist.files.wordpress.com/2010/10/young.jpg