With respect to the 'purpose' ...

I think it has been left unsaid that it is very common to go to an NP
layout and see far too many NP cars - or to a B&O layout and see almost
nothing but B&O cars. And when you look at a picture of a freight yard
or freight train "it just isn't so". The point is that some (many?) of
us get too wrapped up in our favorite railroad and tend to have way too
many freight cars for "our" railroad on our layouts. There are hundreds
of reasons why we do this - a new model comes out and it is killer so we
buy 10 of them instead of 1, a model of a very unique car for out RR is
sitting on the shelf and we just have to have one (or more), we just
love the look of lots of our favorite RR, etc., etc., etc. And let's not
forget that many of us have a 'second favorite' or even a 3rd - that is
also going to encourage us to over-populate. The layouts I'm talking
about are the ones that it takes a great deal of imagination to stretch
the skew enough to justify the number of 'favorite road' cars ... far
more than just the 'need' to represent that one lumber train over Sherman
Hill.
Those layouts that are seriously over-represented with cars from a
small set of RRs just "feel wrong" when you op there. I've op'ed on a
couple where it was so over done that you stopped paying any attention
to the road name part of the car number when trying to locate or spot
a particular car ... because they all seemed to be from the "home road".
We've all op'ed or visited layouts like this.

Having said that - I personally do not think it makes a big difference
whether you take the national percentages and try to match them for your
layout. I -do- think it is important to have enough 'foreign road' cars
that things "feel right" ... but I think that taking it to the nth degree
with respect to the research is ... well to put it bluntly ... totally
nutso and missint the point. That approach is not for me. But that's me.
And that's my opinion.

What I do is this - I try to have a "fair number" of freight cars in
my fleet that are from 'foreign roads' ... but I have to admit that I
have too many from my favorite, 2nd favorite, and 3rd favorite ...
when compared to any national average. But I also think that you should
have some cars that are from RRs that had relatively small percentages
of the national fleet - because even the RRs with the largest percentages
didn't dominate and the actual 'rule' was that there were lots of smaller
RRs that added up to the majority of the freight car pool.
But I also have an advantage over most of you in that my layout is a
club layout and we don't have permanent freight cars on our layout. We
bring the cars we select and set them up before each op session (usually
freight car forwarding with car cards and way bills). We also have op
themes which means that this month the theme might be "Santa Fe in the 50's"
and next month it might be "SP in the 60's" and the month after that it
might be "The SP&S in any year" ... and each member selects freight cars
for that run using a rule of "50% for the home road and the rest from other
roads - if you have them" and so our runs have a different mix and even
different cars from op session to op session - based not only on the
changing themes but also on which members show up for that particular
run and what they chose to put in the case this time.

As had been said on many lists and many times - your methods may vary.

- Jim (Betz) in San Jose