Method to Estimate Car Demand
I love to buy new cars for the layout. As a matter of fact, I have boxes and boxes of cars that are not on the layout, because I:

1. Originally over-purchased cars, and
2. I have, over the years, removed a TON of cars from the layout because it gets too plugged and unbalanced after several operating sessions.

I believe that there is a general tendency to pack the layout with too many cars, and this can impede the optimum operating experience. What follows below is the method I’ve used to determine the how many cars should be on my layout at any given time. I will be discussing averages and assumed percentages, but specific numbers are provided so that the reader can follow my thought process. The percentages can certainly change and vary from layout to layout, but I have used these numbers with success.
After a fun week of hosting multiple operating sessions for attendees to the National N-Scale Convention (which was held in Kansas City in late-June 2016), I had the opportunity to assess the traffic flow on the Mighty MKT. The following discussion presents my criteria and parameters for determining the quantity of cars that are waybilled on the layout.

My rule of thumb is to have each industrial spots in each town half full of car. Of those cars spotted in town, I try to have half of them be pulls and the other half will remain in the town. So approximately 25 percent of the track capacity will be turned over during an operating session. A small town like Mokane on the Mighty MKT has four separate industries with a total of 12 spots, with cars at six of those industry spots. I would set up the waybills to pull 3 of those cars during the session, and have the locals bring in three new cars.
So, there are three factors that enter into the car demand on a layout:

• The number of effective industry spots on the layout
• The percentage of industry spots that would have cars actually located at them (I use 50 percent)
• The percentage of cars that will be picked up from an industry during a given session (I use 50 percent)

In order to determine the demand for the layout, I created a simple Excel spread list, listing all the towns and all the industries on the modeled portion of the layout. For each industry, I then determined the number of “effective” spots available. By effective, I mean the number of legitimate spots that wouldn’t block road crossings of foul turnouts – the number of legitimate spots that would reasonably be used on the prototype. On the Mighty MKT, there are 192 effective (legitimate) industry spots, excluding the industries at Boonville that are worked only by the Union Pacific.

The next step is to estimate the number of cars that would be at an industry at any given time. Using my 50 percent rule of thumb, approximately 96 cars would be located at industries on the layout (note that this excludes the autoracks in St. Charles, the pigs at the TOFC ramps in New Franklin and St. Charles, and the UP industries in Boonville.

Of these 96 cars, 50 percent will be picked up by the locals during the operating session and replaced by new cars brought in by the locals. These 48 (0.5 * 96) cars will be hauled back to Franklin Yard and will go out on the 100-series freight trains (east or west, depending on the direction shown on the waybill for a given car. The layout has seven primary destinations (Boonville, New Franklin, North Jefferson (Westinghouse), Columbia, Mokane, Rhineland, and St. Charles), so each destination will, on average, receive seven cars during a typical operating session. Functionally, the larger destinations will receive proportionally more cars.

The Mighty MKT runs up to five locals during a session:

• Boonville
• New Franklin
• Columbia
• St. Charles
• Mokane/Rhineland

If, during a session, all 48 cars will be pulled and replaced with 48 new cars, and there are five locals, then each local will, on average, be (48 / 5 =) 10 cars long. Again, functionally, the locals destined for the larger towns (St. Charles, New Franklin, Westinghouse), will contain a few more cars. Since I have placed a limit of 12 cars for the locals, this works out about right.

Total Cars on the Layout
If you have stuck with this analysis up to this point, I can anticipate your next question: “Geez, only 48 cars moving on the layout? Why bother?!” It turns out to be a bit more complicated than that. On the Mighty MKT, at any given time, cars can exist in only six states (not including autoracks the TOFC):

• At an industry
• On a local to be delivered to an industry
• In the yard, destined to be placed in a future local for delivery
• In the yard, intended for an east or west off-layout destination
• On a train, intended for delivery at Franklin Yard, thence to an on-layout industry
• On a train, intended for an off-layout destination

If we exclude the full-time off-layout cars that will never be spotted at an on-layout industry, there will be three places an on-layout car can exist:

• At an industry
• On a train destined for Franklin Yard
• Currently on a local OR already in the yard – these items are combined because any car destined for an online industry would be quickly placed onto a local unless the length limit had been met for that given local, or the train had very recently been sent out when the car arrived in the yard.

If we have 48 cars layout-wide that will be spotted during a session, a balanced car flow for the session would look something like this:

• 48 cars currently at industries that will NOT get moved during the session (this is the other half of the cars spotted at industries)
• 48 cars currently at industries that WILL get picked up during the session
• 48 cars destined for industries that are either currently on a local or in the yard. These cars will get spotted at the industries to replace the cars picked up.
• 48 cars on staged trains that will be delivered to the yard during the session. These cars feed the yard and provide the traffic that will make up the next round of locals.

So the 48 cars on the five locals make for an average 10-car train length. And the 48 cars in the five staged trains (101, 102, 103, 104, and Extra 211, a yard to yard transfer) result in an average 10-car cut to be delivered to Franklin Yard per train.