Also, SP had a policy of running long, heavy, slow trains through the tunnels. When you run slow heavy trains, the diesels spend more time in the tunnels. EMD's have higher radiator intakes that suck in hot air from the upper parts of the tunnel. Evidently UP, BN, et al run their trains fast enough that they get out of the tunnels quick enough that the hot air doesn't affect them as much. SP didn't, so they bought tunnel motors.
There's actually a story in Trains Mag a few years back about EMD telling SP they should run shorter faster trains, and SP ignoring that advice. Ergo, they purchased tunnel motors to cope with the extensive exposure to hot cooling air.
In the 90's, SP bought a lot of GE's, which normally have lower cooling intakes (which makes all GE's de facto tunnel motors). That sounds all dandy, but I guess the crews don't like that. According to Doug Riddel, a former Seaboard/CSX engineer and PTJ author, when you walked past a radiator intake with the fan on, it sucked any loose clothing over the grate surface and got it really dirty. I'm guessing he didn't exactly wear seersucker suits, so it had to be pretty messy.
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