In the following video, the body of the locomotive appears to have a lot of empty space. It also appears to have additional connections to the locomotive in front of it. Is this likely a slug? I tried to find information on the 5411 and nothing indicated that it was anything other than a normal locomotive.
https://youtu.be/eFGYTg8sS0M?t=70
No, that's an SD40-T2. The "empty space" is a lower air intake for the radiators positioned above. The SD40-T2 (and SD45-T2) were designed for western roads (SP and D&RGW) that had lots of tunnels where locomotives were overheating. Having the intake lower down let it draw in cooler air.
That's what's called a 'tunnel motor' in many discussions here in the past, including several from people involved with the design.
One significant point of the design was that the radiator cooling fans did not suck air up through the radiator, but pushed it through the radiator core. This is what helped with the 'hot air up high in tunnels' You may have seen locomotives with elephant ear' baffles intending to draw air from lower down in the tunnel -- and heard that these are minimally, if indeed at all, effective for the intended purpose. Pushing rather than pulling hot, less-dense air is part of the 'magic'.
After some research with more specific terms (tunnel motor and the exact model), the confusing part for me was the open space between the intakes. I knew locomotives weren't really known for their wide-open spaces under the hood that would allow for the open area between the intakes.
It appears (correct me where wrong) that they extended the hood back and they may have pushed the air compressor forward a bit. This is from looking at various diagrams found online.
Since the fans are drawing 'from below' it makes sense to make the free area below them as open as possible. This also improves ejection of higher mass flow of cooling air through the radiators.
Now if you truly want to see big openings (albeit with an outsize blower howling away inside) you want to experience the originalspan-bolstered U50s on UP and SP.
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