It is not hard to capture on a model railroad the subtle impression of a prototypical blend of freight cars. Those modelers who achieve it add a new dimension to the realistic operation of their pikes. Somehow, their rolling stock seems to "look right". A markedly realistic appearance is achieved in a model railroad system by building in a definite sense of identity. A significant portion of the rolling stock in use on each line should be from the home road, a practice that mirrors car interchange conditions on most major prototype roads. Also contributing to the familiar image of a railroad is the combination of foreign road cars that one sees on the line. These cars lend a regional tone to the railroad.
According to an old article from Model Railroader, as a general rule, about half of the cars on the property of a given railroad are from the home road. About 25% of the total are from lines that directly connect to the home road. Another 15% are from secondary interchange roads, and 10% belong to private owners or any other railroad in the country.
I have been looking at my rolling stock and have been selling off freight cars that don't fit my mix. I model the SP in the 1940's and 1950's, so I interchange heavily with the Santa Fe, Union Pacific, Western Pacific and D&RGW. Other roads that I would find on one of my trains would be Great Northern, Northern Pacific and Canadian roads (CN & CP). Of course the period modeled would also determine what roads you would find, as well as what types of cars. For illustrative purposes, I will deal with my time frame.
Another aspect of this process, are the types of cars found on the railroad. This same article details a proper mix based on information found the Official Railway Equipment Register. For speaking purposes, let us deal with a 60 freight car roster. For my railroad (the SP) there should be 25Box Cars, 6 Flat Cars, 6 Stock Cars, 6 Gondolas, 3 Hopper Cars, 4 Tank Cars, 6 Reefers and 4 Covered Hoppers. (40% Box Cars, 10% Flat Cars, 10% Stock Cars, 20% Gondolas, 4% Hoppers, 4% Tank Cars, 6% Reefers and 6% Covered Hoppers). You can adjust this depending upon the railroad you are modeling, and the type of goods normally carried. A railroad like the Santa Fe would have more Reefers and one like the B&O would have a larger number of Hopper Cars.
Now the fun begins… Take your railroad and write down what other roads would directly interchange with it. Divide these into primary connecting roads, secondary connecting roads and miscellaneous. This will take some research on the internet for your particular railroad or via The Official Railway Equipment Register. Quite often maps can be found showing interchange points and connecting railroads. For the most part, you should have 50% of your freight cars for the home road, 25% for primary connecting roads, 15% for secondary connecting roads and 10% misc. So in my case, I should have 11 SP Box cars, 3 SP Flat cars, 3 SP Stock cars, 5 SP Gondolas, 2 SP Hopper cars, 2 SP Tank cars, 2 SP Reefers and 2 SP Covered Hoppers. That accounts for the 50% home road and leaves 15 cars for Direct Interconnect roads, 9 cars for Secondary Interconnect roads, and finally 6 cars for miscellaneous and private owner cars.
This may sound complicated but many people enjoy trying to create a prototypical looking railroad. With it getting darker earlier this month, this can be one of those "after dinner" projects that you can play with using only a pencil and paper.
My thanks to Model Railroader Magazine for providing this information to me via an e-mail request.