The Pennsylvania Railroad (reporting mark PRR) was an American Class I railroad, founded in 1846. Commonly referred to as the "Pennsy," the PRR was headquartered in Philadelphia, Pennsylvania. The PRR was the largest railroad by traffic and revenue in the U.S. for the first half of the 20th century. Over the years, it acquired, merged with or owned part of at least 800 other rail lines and companies. At the end of 1925, it operated 10,515 miles of rail line; in the 1920s, it carried nearly three times the traffic as other railroads of comparable length, such as the Union Pacific or Atchison, Topeka & Santa Fe railroads. Its only formidable rival was the New York Central (NYC), which carried around three-quarters of PRR's ton-miles.