Complex sentence: In 1933 PRR completed its electrification of its main line between New York and Philadelphia/Wilmington/Paoli, so most trains to New York Penn no longer needed to change engines at Manhattan Transfer.
Split into simpler sentences: In 1933 PRR completed its electrification of its main line between New York and Philadelphia/Wilmington/Paoli. Therefore most trains headed to New York Penn no longer needed to change engines at Manhattan Transfer.