By the time that CN came to place their next order for another 29 1000 hp MLW switchers in 1957, MLW had made slight refinements to the electrical system of the S-4 and designated it the S-7. Aside from an improved electrical cabinet, S-7 switchers are thus identical in appearance to the late S-4s built by MLW. Units 8206-8234 are the only S-7s built and were delivered in the black and yellow scheme with yellow Railroad Roman numbers and the maple leaf monogram on the cab side.
Subsequently they were repainted into the black and red-orange livery with the CN logo. At least two units (8216 and 8219) received an orange cab but did not get the yellow reflective striping.All S-7s were retired between 1980 and 1985.