Locomotive pooling power

How to railtroads determine wich of their units go into the “pool” to be used by other RR’s? Are any units from the RR’s roster eligable? Just today i saw a new UP GEVO leading a UP SD70M on a CN autoracks train. I would have thoughy UP or any other railway wouldn’t want it’s newest units in the pool, they would rather uese them themselves.

And I thought I finally found someone that spells worse than I do lol

The biggest determinants of what power goes on a train…what power is working at the facility…how much tonage does the train have and can I protect an On Time departure for a scheduled manifest train with it.

Secondary considerations conist of such things as how much fuel does the power have, is the power due for a 92 day inspection soon.

There are several trains that are forwarded to more than one railroad as a solid set of cars and engines. If the train is high-priority, then the “good” engines will be used on it, and it doesn’t matter if the railroad’s newest engines are in the pool for that train.