Following are the topics/questions discussed in this section of the OpSig Primer. You can click on a question to jump directly to that topic. You are invited to address further questions or comments (please refer to question number) to firstname.lastname@example.org. How does one determine the number of cars needed to adequately operate [fill] a layout?
Question 1: I'm unsure how one determines the number of cars needed to adequately operate [fill] a layout. Is there some formula to determine car quantity based on sidings, staging, main line footage, yard tracks, etc.?
Answer 1: I don't know of a 'formula' for estimating car requirements. I think most people keep buying and building cars and putting them on the layout until they realize they are approaching gridlock, then they start winnowing out the weak sisters. But to quantify it logically, one would take a mental snapshot of the layout at a given moment, for example, as of the end of an operating session. There will be x many staging tracks holding x many cars; the online industries will have a car-spot capacity of x, perhaps on average 50% occupied; and there may be interchange tracks holding x many cars. In addition to these 'stationary' cars there could be some cars left over still en route, either in yards, 'off spot' in towns, or even in trains stopped along the mainline. These could be counted by thinking through the events under way or not yet complete at that moment. For example, there could well be cars that came into the yard during the 'day' that won't go out until 'tomorrow's' local is made up.
Another dimension of this would be, what types of cars are needed? Part of the answer comes from what kinds of online industries will be using them (or, what kinds of industries shall we build to utilize our cars)? If your railroad has significant 'overhead' traffic (from offline, to offline) it can be made up of more varied car types, within reason. There are statistics on the types making up the national car fleet, not to mention the fleets of each individual railroad, but that has only limited relevance to your layout's particular traffic flows.
Just to complete the picture, the ownerships represented could also bear some thought. This could be one of those subjects about which one might not wish to know too much, but it is true that while freightcars could go anywhere in the country, in fact they tended to stay more or less close to 'home,' more so for hoppers and gondolas, less so for boxcars or reefers. While perhaps subtle, the ownerships of the cars appearing on a given layout should tend to reinforce the impression that this is New England, say, not southern California. It has been suggested as a rule of thumb that around 50% of cars on line should be 'home' ownership, but this can vary substantially depending upon the type of operation, ie. branchline (more), overhead traffic (less), etc.
Finally, the issue of 'era' bears mention. The appearance of a yard full of freightcars changed gradually over the years so that there are significant differences between the 1940s, say, and the 1980s. Again this is something that may not bother one until he/she becomes aware of it. Not only were 36 and 40-foot cars replaced by 50 and 60-footers, and roof walks largely outlawed after about 1970, but even the road names changed as the merger movement took effect. Cars can be approximately 'dated' by reading the small print indicating built or repainted dates. Again, there is a fair amount of flexibility here, but more extreme anachronisms should be avoided.