Sometimes you will see a railroad company’s train and it will have some other railroad’s locomotives in it. Why is that?
As an example, you might be at a Union Pacific Railway crossing and see a train that has some UP locomotives and a Kansas City Southern (KCS) locomotive. What gives?
This is called “pooling” or “run through power” and has become quite common since the 1980s and the rise of the unit train.
Pooling
Many trains that run today are unit trains, and may start on one railroad’s track and end on another’s. For example, a crude oil train may start on CP in Alberta and end up at a refinery in Texas on Union Pacific tracks.
It is quicker and easier to leave the same locomotives on the train from where it starts to where it finishes. In the above example, CP may leave its locomotives on when the train leaves CP tracks and enters UP territory. This is sometimes called “run through” power, because the locomotives “run through” another railroad’s territory.
Leased Power
It is also common for railways to lease locomotives from companies such as CIT when they have a surge in traffic or too many of their own locomotives out of service. Some of these leased locomotives can spend years working for a railway. It is common for small industrial companies that need a locomotive to move their railcars to lease a locomotive.
Accounting
Railways keep track of “horsepower hours” for borrowed locomotives, by taking the horsepower of the locomotive X the number of hours worked. They “owe” these hours to the other railway, and “pay them back” by lending their own locomotives back for an equivalent period of time. The accounting can be complex.
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