I have a 9x5 layout in N Scale.  The mainline is 41' long.  I have a yard with 4 tracks, 1 is for northbound, 1 for southbound, 1 for local online industries and the other one for my branchline.  On the mainline I have 10 industries and the branchline has 3.  I use the CC&WB method of operation.  There are 45 freight cars on the layout.  I have another 40 in a storage box.  After cycle 4 on the waybill I exchange that car for one of the same type from the storage box.  This way there is a greater variety on the layout.  I also have 2 hidden staging tracks.

My son who was home for the holidays saw this system.  He is a PhD student in math.  After seeing what I was doing he calculated the chances of repeating the exact same train,  No matter if is was a local or a branchline train or a NBD or SBD to staging.  At first he figured the odds to be 300,000 to 1.  A little while later he said he made a mistake, I figured that number was too high, but he said the correct number was 3.5 million to one.  He started to explain how that can be but I lost him in all the mathematical mumbo jumbo.  He said just believe me.

Now I can sleep better knowing the same exact train will never reappear.