According to The Diesel Shop, the mother unit is an SD40-2.When I first looked at my closeup of the mother unit to check the number of axles, I missed that they were C-C units because the trucks were underexposed. I enhanced the brightness of the following photo.
So the slug doubles the number of axles to 12 total to maximize the tractive effort produced by the 3000 horsepower of the mother unit. Doubling the number of traction motors handling the 3000 horsepower also allows the motors to run cooler. I noticed even when they are running light back to the east to get another train to shove over the hump, they run quite slowly. I wonder if they are geared differently than a normal road unit. I have never heard of gearing options for locomotives.
Earlier, when I was driving along the south side of the yard, I caught the end of a train that was working the hump.
At the time I was disappointed that by the time I got parked, etc., there was only the last unit showing. But now I'm glad. It was enough to catch the mother unit, 302, in action, and it revealed the neat railroad yard buildings that are still standing.
|Steven J. Brown posted one of his photos, used with general permission|
Santa Fe GP7u 1311 paired with an unknown slug at Corwith Yard in Chicago - July 19, 1977.
|Mike Breski posted|
RailPictures.Net Photo: IHB 477 Indiana Harbor Belt Railroad Slug at Riverdale, Illinois by Robby GraggRick La Fever The two covers that on either end covered the traction motor blowers and related equipment.
The one in the middle may have been just a slab of concrete.
When used this way, the speed is cut in half but the traction effort doubles.
Marty's post includes an IHB slug.