Here is a good example.
NS and BNSF run a joint operation on containers trains…so, let’s say a NS engine develops a mechanical problem in El Paso.
Its cut from the train, set in the loco repair track…the train picks up another BNSF motor, and leaves without the NS loco.
BNSF is now short an AC4400 for an El Paso to Houston mixed freight, so they “borrow” the NS locomotive after it’s repaired.
Say it takes 24 hours to get here…
Now, we have a NS locomotive running around Houston, no where near NS turf, leading a BNSF freight…and BNSF owes NS the use of an equivalent unit for the same time.
The longer they (BNSF) uses the NS power, the more they owe.
It doesn’t have to be the exact same locomotive, its called horsepower hours, and every quarter, roads try to balance out their accounts with each other.
BNSF may end up “loaning” NS five SD40-2 as pay back, as long as the number of hours x horse power used equals out, or comes close.
It’s hard to cheat, as each locomotive carries an AEI tag, same as cars, and you can follow them from location to location.
Or take where I work…
UP drags a train into the yard, cuts off, and puts the power in the tie up track.
The crew is close to the hours of service, and UP has no need for the power on the train, or a crew to come get it for a day or two.
We might “borrow” the power for that day or two to haul yard to yard transfers…which means we “owe” UP the same number of hours use in equal horsepower.
Now, UP has zero need to borrow our MK1500Ds, and just to pay back a few days of SD70 use would require half our fleet of 25…so once a quarter, we pay a flat rate to UP for the use of any locomotive we borrowed.
You can carry this farther, say the train was a BNSF, with a NS engine, we could borrow the locomotive that BN had borrowed from NS…which explains why you can see a NS SD40-2 running aroun