Paul, now you are talking my early era and California traction focus!
In the teens, 20's and early 30's, industry was centralized in and around south & central Los Angeles. The tracks in LA were not designed to steam standards and contained many typical streetcar and interurban type curves as PE was built from many early small lines. Some of these served the early industries.
Freight loads kinda fell into 3 buckets:
Originating. LA was a major west coast manufacturing hub, so for outbound cars they used their own PE cars (Box, Stock, Flat & Gon)
Interline. In the noted era, much of the traffic was home road. Agricultural, stockcars, LCL, oil drilling, etc. Once again PE cars.
Destination. Shipping manuals flagged customers served by sharp curve trackage, much like those throughout the country where weight or ht/width restrictions existed. Shipping clerks were very knowledgeable back then. Solution was a PE car could be ordered from PE car Mgmt Dept and travel empty to shipper. This was common for loads originating on the nearby SP & AT&SF. Usually though, the inbound was shipped to one of the outlying freight houses and then trucked. You would think that the shipper could do that with PE, SP or AT&SF, but this was the highly regulated era, and if the customer was designated as PE, too bad. (we forget about the business side as it is not as interesting as operations)
In the 1930's industry started moving out and away from core LA, lines were upgraded, and so on, so the unique cars were no longer needed by WWII. But lucky for us, PE cars kept the outside linkage usually to retirement. Probably too expensive to modify and the operational interface was the same as a conventional car.
Rather simplified explanation and missing a lot of detail, but hope it helps. You thinking of knocking out some Sunshine, Westerfield or Owl Mountain PE freight cars?