In the late 1980's General Electric was winning the horsepower war and the new locomotive orders from railroads. EMD was once again behind in technology and revenue. EMD began working on the SD80 series which would achieve 5000hp from a 20 cylinder 710G prime mover, the first 20 cylinder engine since the SD45/SD45-2 locomotive line.
Conrail began to take delivery of a 28 unit order rostered the first units in January 1996. Under agreement with EMD, the 28 units would be built by EMD and finished at the Juniata Shops in Altoona, PA. Conrail numbered these units 4100-4127.
In 1997, Conrail purchased the EMD demonstrator units, EMDX 8000 and 8001 and an order for 108 more units was placed but was ultimately reduced to 28 units. The order was changed with the split of Conrail making the 30 units owned by Conrail the only SD80MACs produced. The demonstrators were added to the roster as 4128 and 4129.
In 1999, the fleet was divided between Norfolk Southern and CSX. Those assigned to NS were renumbered into the 4100 - 4129 series, while CSX allocated units became 800 - 812 and then later renumbered in 2007 in the 4590 - 4602 series.