The SP 4449 (and other "Daylight" steam locomotives) were equipped with Leslie A-200 airhorns. These were not fog horns, they were designed for train and marine use. The A-200 is most commonly associated with early diesel units, mostly E's and F's. The WABCO E2 mentioned above was another airhorn used on early diesel units, though was not as common as the A-200. The A-200 and E2 sound nearly identical. Many late era steam locomotives were equiped with Leslie A-200's, the MILW 261 and SP 4449 are two surviving examples (however the MILW 261 now has a Leslie RS25, slightly different tone). The reason that steamers had airhorns is that a steam whistle is hard to hear at high speed, and with the railroad industry rapidly dieselizing the general public had come to recognize the airhorn as the “train sound”. By most accounts, while in service the SP Daylights used their horns almost exclusively at grade crossings, and the whistles were seldom heard.
Matt F
Train Horn Collector Moscow ID