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[quote]QUOTE: Originally posted by Mookie A question from the curious one. Why would you need more horsepower when the SD70MAC with a unit on the head end and on the rear end, seems to move a lot of coal across Wyoming and Nebraska and on east. Is it because of mountains? I am trying to follow this, but it is a little over my head. Jen [/quote] Good question. For that situation you do not need more horsepower. I have seen 6000 HP units in bulk service (coal, soda ash, etc,) on the UP but it has been used more in intermodal service where speed is needed. The physics is a little complicated but: HP=speed. Getting over a mountain with a coal train is more forgiving without a tight schedule. Sometimes the railroad will add or remove a unit but overall the DPU operation is great because it keeps a lot of tonnage moving without pulling out the drawbar. As long as the train can keep some speed up the hill it will get to where it needs to. One point to remember, you can only get 6000 HP with AC, otherwise the wheels would slip or the locomotive would have to be too heavy for the track. Sorry, more physics. The best way to think of it is like traction control on a car, it keeps the wheels from slipping and it provides the most traction. AC allows a lot of pulling power, even if the speed is low.
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