The Pennsylvania Railroad, an American Class I railroad, was founded in 1846. The Commonwealth of Pennsylvania granted a charter to the Pennsylvania Railroad to build a private rail line that would connect Harrisburg to Pittsburgh. For the first half of the twentieth century, the PRR was the largest railroad by traffic and revenue in the U.S., and was at one time the largest publicly traded corporation in the world. By the 1920s it carried about three times the traffic (measured by ton-miles of freight) as other railroads of comparable length, operating over 10,515 miles of rail line.
At a time when the city of Pittsburgh was emerging as one of the greatest industrial centers of the world, the three primary railroad lines which converged within the city were all owned by separate companies. This resulted in numerous difficulties when it came to coordinating operations between the private railroads, and proved to be a hindrance to further economic growth. Thus in 1920, the Pennsylvania Railroad took over operation of all the western railroads (the previously consolidated Pittsburgh, Fort Wayne & Chicago Railroad Company, and the Pittsburgh, Cincinnati, Chicago & St. Louis Railroad Company), as well as the former "Lines East of Pittsburgh," to form a unified Pennsylvania Railroad Company. This unified company emerged as the foremost railroad company in the United States, and, while its headquarters were located in Philadelphia, the city of Pittsburgh rapidly became its most important hub.
Throughout its 122 year history, the Pennsylvania Railroad Company was responsible for pioneering many operating procedures and inventions which have since become standards within the railroad industry; two such examples which remain industry standards include the vestibulated passenger train, and the use of position-light signals. Moreover, the PRR was truly unique among American railroad companies, as it designed and built most of its steam locomotive classes itself in its Altoona Works. The PRR is believed to have been the fourth most prolific U.S. builder of steam locomotives.
Ultimately, the Pennsylvania Railroad merged with its rival, the New York Central Railroad, in 1968 to form the Penn Central Transportation Company, which was later transferred to Conrail in 1976. In 1999, the Norfolk Southern Railway and CSX Transportation acquired Conrail in approximately equal portions. Norfolk Southern now owns most of the former Pennsylvania Railroad, including the Harrisburg to Pittsburgh segment of the old Pennsy Main Line across Pennsylvania. Amtrak currently owns the remaining segments east of Harrisburg.