Thin as a rail

Origin

The phrase draws its striking image from the most literal sense of a 'rail'—a long, slender bar of wood or metal. Whether picturing a fence rail, a stair banister, or a railway track, the common characteristic is its relative lack of width compared to its length. This direct, visual comparison for extreme thinness began to take hold in the English language in the 19th century, especially with the rise of widespread railway networks which made the image of the thin, straight rail even more ubiquitous. It’s a simple, vivid metaphor that immediately conjures an image of almost skeletal slenderness, making it easy to understand and quick to adopt into everyday speech.