The Monongahela Connecting Railroad (AAR reporting mark MCRR) or Mon Conn is a small industrial railroad in Pittsburgh, Pennsylvania. It was a subsidiary of the Jones & Laughlin Steel Company and a large portion of its work was for its parent company, though it also served other industries along the line. The railroad is possibly best known for its Hot Metal Bridge which was used to carry molten iron across the Monongahela River from J&L's Eliza Furnaces to the Bessemer converters and rolling mills at J&L's South Side facility. The railroad was also a dieselization pioneer, buying many early diesel locomotives from General Electric and other manufacturers. The railroad is still in existence, but in much reduced form. The Hot Metal Bridge has been converted to a two-lane automobile bridge, and the railroad serves a few small industrial customers along one bank of the river.