My research shows that - for the most part - if a tank car is lettered
for the RR (any RR) ... that it is in "company service" and not available
for general loading. Examples would be cars delivering fuel oil to
the railroad itself rather than to an industry on that same RR.
Similarly, it is rare to find a picture of an RR lettered tank car that
is not "sitting on some RR fuel service track". I have been collecting
pics of all sorts of freight cars for several decades now ... and have
yet to see a pic of any RR lettered tank car that is even in a yard -
much less in a train out on the line. They 'all' seem to have been
taken when the car was sitting at some fuel facility or next to the
roundhouse/diesel facility. The few pics I have of an RR lettered
tank car in a train ... it is 'alone' rather than being coupled to other
tank cars going to/from some place to load/unload it.
I have not found any "era differences" to this 'detail' either.
Due to this fact(?) I own very limited numbers of tank cars that are
lettered for "my roads". Most of my tank cars are lettered for other
than any RR - with UTLX and GATX being the most common.
Sorry - I have no intelligence about ATSF models (other than pics of
them showing the way they were lettered/marked).
- just saying ... Jim