You know what it looks like… but what is it called?
TAKE THE QUIZ“fish-belly”
The following 2 entries include the term fish-belly.
fish belly rail
noun: a short-span rail supported on chairs and curved on the underside so as to be deeper at the middle than at the ends of the span
fish belly sill
noun: a side or center sill shaped like a fish and used in railroad car construction
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Merriam-Webster unabridged