Complex sentence: The more important of the two companies, the American Line opened Philadelphia's first immigrant station at Pier 53 at Washington Avenue with support from the Pennsylvania Railroad, which sought to increase its dominance of the U.S. passenger and freight railway market.
Split into simpler sentences: The more important of the two companies, the American Line, opened Philadelphia's first immigrant station at Pier 53 at Washington Avenue. The station was also supported by the Pennsylvania Railroad, which sought to increase its dominance of the U.S. passenger and freight railway market.