thin as a rail - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Very slender, as in I do not know why she's dieting; she's thin as a rail already. This simile, which uses rail in the sense of “a narrow bar,” has largely replaced such other versions as thin as a lath or rake, although the latter is still common in Britain. [Second half of 1800s]