After all, the originator is sending loaded cars all the time to their
destinations -- So what logic would there be for hoarding cars?"

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To protect other business. Coal isn't the only thing that gets loaded
in hoppers. Suppose the hypothetical East & West railroad has a trap
rock quarry, but owns no hoppers. To supply cars, they grab any empty
coal hopper ostensibly to load it toward home... but the railroads
didn't have a lot of control over where the customer actually loaded
the car to, and so after the customer loaded all the coal cars to
points on the E&W, they ask for more empty cars, and the E&W obliges
with more empty foreign coal cars. To the car owner it looks like
their cars are going to the E&W and never returning, because that's
exactly what is happening.

The typical response was to complain to the AAR That the E&W should
buy sufficient hoppers to cover their traffic, but the AAR did not
have any authority to order anyone to buy cars... they best they
could do is issue these car service orders that directed empties be
returned immediately rather than re-loaded.

Dennis Storzek