The description "a huge squirrel cage fan that partially sticks out of the left side of a locomotive" sounds more like traction motor blowers on some early electric locomotives than like those on modern diesels. Diesel tr. mo. bls. tend to use air that is already inside the carbody, and do't show externally.
If you go to George Elwood's "Fallen Flags" rail photo sites
http://rr-fallenflags.org/
and go to the "Operator's Manuals" section you will find a number of operator's manuals for assorted diesel (and some electric) locomotives: most of these will contain diagrams showing the location of various pieces of equipment on the locomotive, which will show where the t.m.b.s are. (Older locomotives-- first generation diesels, from the 1930s 1940s 1950s -- had separate blowers, either one per motor or one at each end of the locomotive, either driven directly from the engine or run by separate electric motors. I think more modern ones usually have a single blower, with ducts carrying the air to the traction motors and other equipment that needs cooling air.)
Since the 1980s, locomotives have had on-board computers monitoring and controlling many aspects of their performance. My guess is that (in order to reduce power needs, and so fuel consumption) blowers are now computer-controlled, and only run when (and at the speed) needed.