Oops. My calculation was wrong. If you have ONE box car on

your layout at a time, and .14% of the fleet is Monon, then

the expectation is 40% that at least ONE of those days during

the year, the car will be a Monon car.

But if you have 100 box cars on your layout, then the expectation

on any given day is 13%, while the probability that no Monon box car

will show up within a year is .00000000000000000000619%. In other

words, there's a snowball's chance in Hell that no Monon box car

will show up.

If you have a layout with 500 box cars, then the daily chance of

at least one Monon box car showing up increases to 50%. And the

chance that at least one will show up within a week's time is

more than 99%.

Just because freight cars are proportionally few, doesn't mean

they are necessarily unusual visitors. I think that one should

do this simple calculation based on the number of cars of that

type on your layout, starting with your "average" of say, 25

cars from "eastern" railroads. Then you may find you really have

90 or 100 models that have a high likelihood of showing up every

5 or 10 operating sessions. You can calculate probabilities for

each and then proportion the number of slots (e.g. 25 x 10 days)

to get a nice, variable but realistic mix of freight cars. Or

you could just program the probabilities into your computerized

switchlist program, and have it produce a theoretically realistic

mix every day...

And therefore, the probability that NONE of the Monon box cars

showed up in a year's time in any particular location is less

than 6%... Or put another way, there's a 94% probability that at

least one of the Monon box cars would show up within a year, assuming

that all box cars are equally probable. (Given your theory of random

box car assignments.)

Tim O'Connor <timboconnor@...> -->> NOTE EMAIL CHANGE <<--

Sterling, Massachusetts