Here's a useful WWWebsite for questions like yours:
Locomotive rosters for most North American railroads, reasonably up to date for the bigger ones. (Kansas City Southern is one of the "Class 1" ones.) Plus assorted other locomotive information (not 100% accurate -- this IS the internet, after all -- but over all pretty good.)
For a long time (??1960s through 1980s??), KCS bought only locomotives built by Electromotive (then a division of GM, now an independent company). More recently KCS and its Mexican affiliate seem to have been buying equally from Electromotive and General Electric (the two current builders of mainline freight diesels in North America). Earlier-- when first converting from steam to diesel-- KCS had a mix, including some Fairbanks-Morse "Erie-builts": Fairbanks-Morse used GE electrical equipment on these locomotives and subcontracted their assembly to GE's Erie (Pennsylvania) locomotive plant in the late 1940s. The F-M engines in them were high-maintenance, and KCS had the actual diesel engines in the locomotives replaced with EMD engines in the 1950s, before trading in the locomotives in the 1960s.
I have a booklet of locomotive rosters (DPA-LTA publications) from 1973. At that time KCS had 212 locomotives, all built by EMD: 90 dated back to their initial dieselization (they were built between 1946 and 1953), 122 were newer (1959 and later). They had 83 end-cab switchers (an unusually high proportion for mainline railroads at that time): 39 NW-2 and SW-7 from their first generation diesel fleet and 44 SW-1500 built between 1968 and 1972. Their biggest and newest power at that time were 56 SD-40 and SD-40-2 (six-axle, 3000h.p. freight luggers) built between 1968 and 1972.