When I was a kid in Michigan and lived near Trowbridge where the Grand Trunk and Pere Marquette crossed, I used to see all the different cars from all the different lines go by. But the locomotives were always of Grand Trunk and Pere Marquette. Now they seem to be from many different lines. Why and when did this happen?
Up until about the mid-1960’s, locomotives stayed on their home roads except for a handful of passenger power pools (South Wind, Cities Streamliners, and a few others) and leasing out of old power to other power-short roads. In the mid-60’s, railroads began running through freights past interchange points as a way of expediting service. Each road would contribute power to the pool on an agreed ratio and the power ran through with the train. An example with which I’m familiar is an EL/RI through freight between Silvis and Marion, OH. PC also pooled power with RI and ATSF on through freights over the Kankakee Belt. There’s obviously a lot of other factors involved here but I’ll leave that to the more knowledgable contributors.
Also, as railroads began to be more competitive, they also looked to cut costs. Some traffic is seasonal, like grain. Thus some months one railroad may find it has surplus power that must be maintained to cover surges in traffic at other times of the year, and it makes sense to rent the power to the railroads that may be experiencing a surge right then.
IIRC, from previous posts on the topic, the run-through practice is common, as is “loaning” power. What gets complicated is that money doesn’t always change hands - if RR A borrows an engine from RR B for X horsepower hours then they will pay RR B back by sending an engine over for the same number of horsepower hours. Of course, it gets pretty complicated, since it’s not likely that an engine will arrive at an interchange point exactly when the debt is repaid, so now we have RR B ‘borrowing’ an engine from RR A and the shoe is on the other foot.
Add to that the plethora of leasing companies, and you get some pretty colorful consists.