Probably the most well known switcher from the Conrail roster, the 121 EMD SW1500's were a 1,500 horsepower variation offered by EMD. Inherited from the PC, RDG, IHB and IU, all but five (which were retired and sold in 1982-1984) made it from 1976 to 1996 when a few were retired and sold off to regionals and/or shortlines.
During the split in 1999, CSX and NS both got their share of this popular EMD switcher.
CSX has since traded theirs to LLPX in exchange for higher horsepower road units. As of 2012, NS retired all of this venerable switcher except 2120 and 2121, which were rebuilt by NS to RP14BD Genset.