/ Dictionary / Index P plain rail: Meaning and Definition of Find definitions for: plain' rail" Pronunciation: [key] (in a double-hung window) a meeting rail equal in thickness to its sash. Cf. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. Plain People plain sail Related Content Daily Word Quiz: secular Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language