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Milwaukee Road Matt

You are just talking about the tank cars, right? They weren't in between locomotives. The green/white ones?

Occasionally, I would see them in mountain trains.

I have heard they were used for two reasons:

first, to lessen the need stop for refueling.

secondly, (this is just what I heard) is that obviously fully fueled locomotives will have better traction. In an effort to keep the units fueled and "heavy," they would place these cars in the power lash-up to maintain a fuel level.

I forgot to mention this was before the rise in AC power!

I don't know anything about the air repeater cars.

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The railfan side

My pictures

Between two locomotives SD40-2's mainly, the reason that I always read was high fuel cost and the lack of stations along the routes for the engines to refuel.

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Milwaukee Road Matt