Shawn,

You wrote:

Where would one go to find information on the interaction
between railroads and the automobile industry in the 1940's
and 1950's? My interest in this actually goes a decade beyond
this time frame, but I'll keep the question within the limits
of this list's charter.

Being as how the Southern Pacific and Cotton Belt did a huge
amount of business hauling automobiles and auto parts over the
years, I've been attempting of late to discover the basic "in's
and out's" of how all this freight was moved:

* Where the on-line parts manufacturers were located.
* Where the on-line assembly plants were located.
* What freight cars were assigned where - and when - to handle
the traffic from the various plants.
* What documents exist - if any - that show automobile and auto
parts car "pool" assignments.

Where can I go to find publications or documents that have this kind
of detailed information? Does any such document even exist? Would
there
be some kind of government publication that contains this information?

There may be a list of plants in an auto company's 10-K filing to the
SEC; I have no idea of whether they would still be available for the
years you want from the Government or the Company, but the worst answer
you can get from either would be "no." Another place to check for 10-K's
may be the historical collection at a Business School like Stanford.
Whether the LA Public Library has old 10-K's in their stacks is another
place to explore.

In terms of freight cars assigned, the ORER footnotes can yield the type
of Auto Parts loaded - e.g. in the January 1938 ORER, per footnote R,
NYC series #85000-85159 were equipped with racks for loading finished
closed automobile bodies; per footnote S, NYC series #85160-85234 were
equipped with racks for loading Automobile tops. Unfortunately for SP
buffs, that same 1/1938 ORER's footnotes to SP cars do not distinguish
what type of auto part is carried by the relevant series.