This is an apparently evergreen topic, as I have posted about it a couple of times, but continue to get inquiries about it. My most recent post on the subject is here: https://modelingthesp.blogspot.com/2021/08/cars-on-layout-for-op-session.html , from five years ago. But let me re-visit the area one more time.
Most expressions of interest have been about choices of cars, and the related topic, how the layout car population changes or continues from session to session. There is a general pattern, that a car coming onto the layout via delivery by the Guadalupe Local is then switched to its destination industry, in that session or the following one. It then may be picked up and taken outbound in the next or next following session.
But there are also cars that get added to the layout before a particular session, maybe by being placed directly at an industry, and then picked up in that or a subsequent session. And on occasion a car that has resided at an industry for a couple of sessions may be removed by “0-5-0” before the next session. Thus there is not a rigid or unchanging rhythm to car movements.
So what’s going on? Don’t I have a plan or a system? Well, if you have read this blog very often, you know that I would be bound to have a system. It’s just not inflexible. I always cringe a little bit at the term “play value,” but this is a good example of what we mean by it. I want each session to be interesting and varied, and to include a wide variety of freight cars.
So if we look at the photo below, just taken today at Shumala on my layout, showing an assortment of cars being readied for an upcoming operating session, what do we see? I will come back to the details, but these seven cars include a pair of SP gondolas at right, an Associated Oil Co. tank car and a New York Central box car to their left, and behind them, box cars owned by the Rock Island, Buffalo Creek Flour, and the Cotton Belt. This does reflect an aspect of the variety I want.
Now what do I mean by “variety?” I mean prototypical variations in the car population by railroad. To me, there are two parts to this aspect. First is home-road cars; second is what railroaders call “foreign” cars, those owned by all railroads other than the home road.
There are extensive data for any railroad about home-road cars, in the form of yard and train photographs. Ideally, one hopes to also find less “snapshot” kinds of evidence, in the form of conductors’ time books, yard lists or wheel reports, though disappointingly few such documents survive. For the railroad I model, the Southern Pacific, data show home-road cars to be around 25 to 35 percent of all cars (for example, see the analysis here: https://modelingthesp.blogspot.com/2011/03/modeling-freight-traffic-coast-line_11.html ).
For foreign cars, we have the benefit of a very interesting idea, the Gilbert-Nelson hypothesis (which I’ve described in detail previously: see https://modelingthesp.blogspot.com/2010/12/choosing-model-car-fleet-2.html ). This hypothesis, well supported with data for trunk line or long-haul railroads, is that freight cars appear in proportion to their percentage in the national freight car fleet. Thus our main tool for choosing foreign cars is data for that national fleet.
My most-used version of this tool is the graph below. What it shows is fleet size for the “top 15” railroads. This graph is for 1950, one that I have shown previously in several places, showing the size of car fleets, minus coal and ore hoppers. (For a defense of that adjustment, see this post: https://modelingthesp.blogspot.com/2021/05/proportions-of-freight-car-fleet.html , or my article in Model Railroad Hobbyist for December 2011).
A few things to point about this graph. First, the five biggest fleets (other than SP) are the PRR, NYC, ATSF, Milwaukee and B&O. I try to make sure that cars from three to five of these roads participate in every operating session. Second, notice that PFE shows up here because of its fleet size; I do indeed operate a considerable number of PFE reefers, as they provided SP with its reefer needs. Rock island, with a car contained in the photo at the top of this post, is well down in this graph but obviously one of the largest freight car fleets in the U.S., and also an interchange partner with SP.
What about that Buffalo Creek car? In my era, the Buffalo Creek Railroad owned exactly 500 box cars, all leased to Buffalo Creek Flour. Isn’t that far too small a railroad for me to have one of their cars, after looking at the graph above? Here I like to quote a comment from my friend Tim O’Connor, that each “tiny” car owner individually is invisible, but collectively they rostered over 5 percent of the national fleet. So a car of a some tiny railroad really should show up occasionally.
Beyond the intent to include about 30 percent of an operating session in SP cars, and to include cars of the “Big Five” shown above, everything else depends on the needs of shippers. I have documented previously my overall framework for determining these needs. (See that post at: https://modelingthesp.blogspot.com/2011/11/operations-demand-based-car-flow-2.html .) This forms the background for choosing car movements in any one session. If all this isn’t clear, please contact me with questions.
Tony Thompson
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