That's one of those "how long is a piece of string" questions, the answer could be 5, 50, 100, you name it. It depends on too many variables, steam, diesel, era, branchline, mainline and so on.
You'll need to be a lot more specific with your question to get any intelligent answers unless you just want some random numbers posted.
Mark
RMR
In N-scale I run unit coal trains that are 32 cars long. Well, I used to before I sold all of my Kato coal cars to raise cash for track. On my layout, that length feels like a "long" train, especially if you don't highball around. Jamie
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How long are your coal trains?