I live near a Norfolk Southern main line, and use an at-grade crossing with the tracks pretty frequently. About a week ago I was stopped at the crossing and noticed the train was pulled by a Norfolk Southern engine with a BNSF engine in tandem. It was an unusually short train (I didn’t even get a chance to put it in park while it went by) but the cars didn’t seem to be an unusual mix. Is it common for US class 1 railroads to use each others’ engines? If BNSF just sold the engine to NS, would they use it before painting it in their own livery?
The BNSF engine could be on a temporary lease to NS. They wouldn’t repaint the engine for a lease.
Another possibility is that the engine was part of a run-through agreement. In this case, BNSF and NS would have come to some sort of agreement for whose engines get swapped out where. The overall advantage of this is that cars get moved faster and more efficiently between the two railroads (less time sitting to switch engines in and out), so it can actually benefit both railroads to share their engines.
So what you’re saying is that if BNSF and NS trains were going to the same place and arrived at a classification yard at more or less the same time, they could be combined into one big train with both companies’ locomotives pulling, and then broken apart/reassembled into their own trains at some future classification yard?
They might be combined and then broken apart later. They might have certain runs that regularly go back and forth into each other’s territories. Rather than constantly switch cars at the border between their two railways, they might agree to share the load, with one engine from each running back and forth between their two territories. The both end up splitting the overall freight load but now there’s no delay for them to switch engines around. There are a lot of different possibilities.
A run-through agreement just allows one railroad’s engines to continue into another railroad’s system so that they can just haul the freight through faster and don’t have to wait for engine changes. There are a lot of different reasons why railroads might sign run-through agreements with each other. Looking at their maps on wikipedia, BNSF is mostly west coast and central, where NS is east coast. Having run-through agreements between the two companies makes a lot of sense. Any of their customers who ship coast to coast or at least between the two territories will be happier since the freight arrives at its destination faster.