David Smith wrote:

I searched the STMFC archive and couldn't find any mention of longitudinal hoppers . . . is it a ballast car only or would it have had other uses?

To mention just one railroad, SP bought nearly all longitudinal hoppers until the 1960s, and they were equipped with either "Hart Selective" or "Enterprise" ballast-distributing outlets. They could also, of course, be used for other mineral cargoes as needed.
The name comes from the rotation axis of the dump doors--it was longitudinal rather than across the car, as with "conventional" hoppers. You will find the term used in Cycs.

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