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Re: fleet composition
I REALLY think this whole issue can be greatly simplified for most layouts.
Let's assume you use good "switchlist" software to generate train switchlists
and random car assignments [aka waybills] (following your realistically programmed
car assignment criteria).
Now let's take one example -- A daily heavy freight that primarily carries
mineral products in covered hoppers, originating at say, Green River WY.
Now suppose you own 30 model cars for this service, and your operating rules
allow a train length of 25 cars. Ask a simple question:
HOW MANY DIFFERENT TRAINS OF 25 CARS CAN BE CREATED FROM A SET OF 30 CARS?
Well, it turns out, a LOT. In fact, you can create 142,506 different combinations
of freight cars from this set of 30 cars, chosen 25 at a time !!!
You can do this calculation for any train, or mix of trains, or mix of cars
that you wish. This is the famous "M out of N" combinatorial math formula. If
you had more cars (100) and a shorter train (20) you can be sure that you will
never, ever, ever have the same set of cars in the train, no matter how many
times you run that train, over a period of more than 10,000 years. :-)
Here's the calculator -- have fun!
P.S. If you play bridge (13 cards out of 52), the calculation shows there are over
600 BILLION possible hands. I guess that's why people find cards so interesting!