Choosing a model car fleet, Part 2

As mentioned in the previous post with this title (http://modelingthesp.blogspot.com/2010/12/choosing-model-car-fleet.html), I adhere to the idea of proportioning a model car fleet to prototype proportions. In the previous post, I used the example of PFE cars. Today I would like to comment on a much broader issue, the proportions of cars other than SP and PFE (called “foreign” cars by railroaders).
     The basic approach nowadays to this problem is called the Gilbert-Nelson approach, based on ideas first developed by Tim Gilbert and Dave Nelson. They had looked at extensive prototype information, particularly conductor’s time books, and realized that there was a pattern: many cars present in trains were numerically in proportion to the size of the owning road’s fleet, regardless of where in the country the data originated. Of course, as they fully recognized, this can only be true of free-running cars like box cars, flat cars and gondolas which are not specially equipped, and is likely true only on main lines. A coal branch, for example, will obviously be quite different.
     They also recognized that certain factors can distort the general pattern. For example, interchange requirements or pool agreements can change the data; so can “hostile” or competitive relations among railroads. It’s well established, for example, that the intense rivalry between PFE and Santa Fe’s SFRD meant that empty cars were always sent back to the other company and not reloaded as foreigns. On the other hand, the legally required preference of SP interchange with UP at Ogden would increase each road’s presence on the other. But absent such considerations, free-running foreign cars would be expected to follow Gilbert-Nelson.
     This means that one can collect data on the larger fleets of freight cars in the U.S. and use it as a rough guide to foreign cars on an SP layout. These would include the following major players:

     These data are for 1950, and are modified from total car fleets by removal of ore cars, hopper cars, and ballast cars. As an SP modeler, the likelihood of coal-road hopper cars on my layout is pretty small and I neglect it. I have also separated out the PFE cars from SP and UP. So this figure shows the major railroad ownerships I need to model, and once beyond the “top six” in this diagram, numbers don’t vary greatly.
     How might these railroads be affected by the special considerations mentioned above? It’s known that SP had friendly interchange relations with NP, RI and IC (at far ends of the system) in addition to the UP relationship mentioned above. Conversely, ATSF and WP were rivals in the far west, while MP was a strong rival of T&NO in that territory.
     The final question I’ll raise today is the proportion of home road cars, vs. foreign cars. This does not seem to have a universal answer, and Gilbert-Nelson found different proportions in different places. There is an old rule of thumb, that home road cars may be a third to a half of all cars, but that seems too high for SP in California, based both on photos and conductor time books. I will probably settle on one third, which is perhaps a credible compromise.
     Some discussion of this issue, predating Gilbert-Nelson, can be found in an article I published some time ago, copies of which can be obtained from the NMRA library. It is the source of data for the graph shown above. Here’s the citation:
Anthony Thompson, “Railroad Freight Car Fleets," in Symposium on Railroad History, published by A.C. Kalmbach Memorial Library, NMRA, Chattanooga, TN, 1990, pp. 27–44.
     In future posts, I will explore this topic in more detail.
Tony Thompson