6491 was built in September 1969, #35478, as St Louis-San Francisco 930.  It became Burlington Northern  6678, Class SD-45, in December 1980 and sold to Soo as 6491 in March 1987.  It was retired in June 1992,  It was sold to Morrison-Knudsen Corp as 9528 in March 1993 and sold to Wisconsin Central Ltd as 9528 in 1997.  It was renumbered 6638 and then rebuilt as 7638.

6492 was built in November 1971, #5794-10, as BN 6552, Class SD-45.  It was sold as Soo 6492 in December 1986 and sold as National Railway Equipment Co 6492 in June 1992.  It was rebuilt by MK Rail on June 6, 1994, as Southern Pacific 8587 and became Union Pacific  2663 on January 29, 2003.