I'm certainly no expert on the quantities of a given car that is made and car distribution, but personally I don't think quantity is the only criteria as to whether a specific prototype of car would be relevant for a layout. Obviously there is some sort of threshold as to what would be an appropriate cut-off below which it would be very rare. But the distribution of cars based on percentage of the national fleet is in regards to free-roaming cars (like box cars).
Are tank cars free-roaming?
My understanding is that they had to be returned empty to their home location after unloading. As such, they are in a sort of captive service. Starting from the home point, the cars from a particular industry would radiate out, with them being most common near their starting point, and rarer the farther you go. A given car might end up going to hundreds of consignees over time, and may not appear in a specific location frequently, but other cars of similar types would.
If an industry on your railroad contracted with a specific dealer, then you would probably have a high percentage of a particular group of tank cars. But I suspect that they shopped around, and thus if you have an industry that needs sulfuric acid, you would see a variety of sources, and thus a variety of prototypes over time.
It's partially because of the fact that these are not truly free-roaming that they would need to build fewer cars to meet their needs. If a dealer knows they ship on average 'X' number of loads daily, and that the average length of trip is 'Y', then they know they only need 'Z' number of cars. If your cars could be confiscated at any time, then it takes much longer for your car to get home.
For tank cars I believe it's much more important to look at the ladings needed on your layout, combined with those served by through freights that would pass through your layout.
In my case, I know I need oil, gasoline (Gulf, Esso), sulfuric acid, LP gas, possibly butane, and probably chlorine and ammonia at the very least. In addition to Gulf oil, there are dealers for Texaco, Mobil, Tidewater, and others. They aren't located online, but needed to be relatively close and although I haven't confirmed it, at least some are likely to be located in Hartford. Thus tank cars of all of those dealers, and a variety of types are likely.
So while I agree that most of them (at least in the immediate post-war era) were probably plain black tank cars, they would have been of a variety of types of cars, including acid and propane-type cars. If you're modeling a specific location that doesn't get any through traffic, then your needs might vary.
I've also discussed car rarity on my site a number of times, and had quite a few discussions with friends on the subject. Granted, this is based on my needs, and I have 4-5 daily through freights, split between two separate mainlines. But I know a lot of people that set a roster size based on the assumption that all of their cars are on their layout (or in staging) all of the time. So, say 100 or 200 cars. Of that they designate a certain number for 'rare' cars. Perhaps 5%, or 5 to 10 cars.
So the mix of common to rare cars overall is reasonable. But if you only own those 5 'rare' cars, and they are always on your layout, at least every other session. Then they have become common.
I love freight cars. And there are a lot of rare cars that I can legitimately run on the layout, just not all at once. Each session I randomly select cars based on the waybills I've generated, and a small percentage are rare cars. But any specific rare car might appear on the layout once every 10 sessions or so. I'm building shelves so others can enjoy other cars that I've modeled but aren't in the current session.
Oddly, I'm finding that the reverse is true. Initially I was using a threshold of 5,000 cars, and I'd build roughly 1 per 5,000 cars. So I determined I needed 15 to 20 X29 box cars to keep the balance 'accurate.' It wasn't until I realized that I really only need enough common cars for the maximum that would appear in a given operating session. Maybe 6-10 tops. This is because I don't think people will remember a road number from one session to the next (unless is 1, or 666, or something that stands out). I do think they'll notice that bright orange Waddell hopper though. So as my roster has been evolving, the mix is skewed toward the rare cars, so I can have a proper mix during a single session as well as over time.
I'm extremely happy that we're seeing additional tank car prototypes from a number of manufacturers, both RTR and in resin kit form. My only complaint is that the build dates on many of them are at the end of my modeling era. Since I will be setting up the layout for specific years, including equipment and industries, and the primary focus is intended to be 1947, I would prefer slightly earlier prototypes. But I'm not the only modeler around here, and there are plenty of cars that others want and need.
Modeling the New Haven Railroad 1946-1954 | http://newbritainstation.com