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Car

A contract or unit of trading. Originally, one contract, or "car," was the quantity of a commodity that would fill a railroad car. See also lot.

Source: https://www.cmegroup.com/education/glossary.html

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Why do some people refer to futures contracts as "cars"?

I always wondered why futures contracts are sometimes referred to as "cars". A little Googling and I found this. I thought some of you might wonder the same thing.

Black Monday: The Stock Market ... - Google Books (