Complex sentence: This pontoon bridge--the first railroad bridge in Chicago--was completed in 1852; it allowed trains to access the railroad's new Wells Street Station and subsequently industry on the north bank of the Chicago River as far as the Ogden Slip and Navy Pier.
Split into simpler sentences: This pontoon bridge--the first railroad bridge in Chicago--was completed in 1852. This bridge allowed trains to access the railroad's new Wells Street Station and subsequently industry on the north bank of the Chicago River as far as the Ogden Slip and Navy Pier.