I agree - there were/are exceptions to any "rule" about freight
cars that has ever been presented as "fact" - including mine.
And there certainly were some tank cars that were lettered for
a road that were used in general service (or even in captive
service for non-RR related traffic).
I was trying to talk about the "most common" use of them and
intentionally avoiding the exceptions.
The fact that tank cars lettered for a specific RR are listed in
the "in revenue service lists" does not surprise me in the least.
If the car has to travel on foreign rails it would have to conform
to all of the interchange rules/regs - at least one way to do that
would be to list it as in "revenue service" ... but listing it that way
doesn't mean that it was actually available for general use.
At least one other explanation for listing them in revenue
service is that the RR could "re-purpose" the cars in the future if
they wanted to ...
My suspicion is that when a tank car was 'actually in company
service' that it carried a waybill that went to a specific destination
(such as an offline refinery/whatever) ... even when it was empty.
And then after it was loaded it carried a waybill that would deliver
it back to its home road as quickly and directly as possible.
Since there are/were refineries "every where" the likelihood
that a tank car would actually leave home road rails for any long
distances was probably pretty remote. But it was probably quite
common for it to leave home rails long enough to be delivered
and picked up from a refinery/what ever if that refinery was only
served by a 'competing' RR.
Let's not forget that the RRs themselves were/are significant users
of fuel! But even at that the percentage of cars moving/traveling
on any one day for that use is usually considerably less than cars
in other fuel related revenue service.