Often called the "Pennsy", the Pennsylvania Railroad was founded in 1846. It was headquartered in Philadelphia. For the first half of the twentieth century, the Pennsy was the largest railroad by both revenue and traffic in the US. It had merged, owned or leased over 800 other railway lines. It ran on 10,515 miles of railway and carried almost three times as much traffic as other railroads of roughly the same size. It's closest rival, the New York Central carried only three-quarters of the "ton-miles" that the Pennsylvania carried. The PRR was an important financial force in the US, being the largest publicly traded corporation in the world. It's budget was larger than the entire government of the United States and it had a workforce of close to 250,000 people.
Other businesses were in steep competition with the PRR including the Erie Canal and the National Railway and great deal of commerce of the time was being carried through New York and Baltimore that could have possibly been funneled into Philadelphia. The state, which owned canal systems were able to enact a tax on railroad tonnage and this helped to make the canal systems more profitable. In spite of the competition, by 1910, the PRR had reached it's full growth potential.
Between 1920 and 1930, the PRR electrified its lines from Washington DC to New York by way of a direct current (DC) 650 volt third rail. This made it possible to run locomotives and passenger cars with electricity.
As the automobile became more common it became less common for passengers to travel via trains and that aspect of railways began a serious decline. Changes in track sizes, and loading bin sizes also caused a decline in railway commercial business as well. Although Railways had initially provided a reasonably comfortable mode of travel and migration into new areas, their frequent lengthy stops and the ability for a family to travel within its own automobile soon eclipsed the value of the train as a means of passenger travel. As the American commercial trucking grew, the ton-miles of freight trains also decreased.