@JimBreen

I don’t think "彼女はレールのように痩せていた" is a good translation for the English. I've added the new one "彼女はガリガリにやせ細っていた."

Perhaps you might be interested in [#10177420] as an example for a rail.
レール温度が上昇した影響で、列車は３０分遅れで到着した。

> 彼女はレールのように痩せていた"

This is a literal translation. Perhaps it should be deleted from tatoeba.org.

Note that "rail" means something like a handrail in this idiom.

Google images: rail fence

https://www.google.com/search?q...w=1226&bih=690

JMdict's レール entry currently has 3 senses: "(n) (1) rail (track); (n) (2) curtain rail; (n) (3) groundwork; spadework; preparations; arrangement"
This sentence pair is being used as an example of sense 2. I realise it's very 直訳 but I wouldn't like to see it deleted without there being a suitable replacement.
The new sentence added by bunbuku is a good one for sense #1. I have suggested a translation for it.

I suspect that the origin of "thin as a rail" wouldn't be referring to a curtain rail, so it wouldn't really be a good match, I think.

Bunbuku, would you like to create a "curtain rail" sentence?

https://www.google.com/search?q...w=1226&bih=690

https://idioms.thefreedictionar...thin+as+a+rail

To me, "thin as a rake" is much more common.

リーダーズ+プラス has:
(as) thin as a rake [lath, stick] 〈人が〉ひどくやせこけて.

For me, too, "thin as a rake" seems much more common.

For fun here are NGRAM comparisons

All English
https://books.google.com/ngrams...26&smoothing=3

British English
https://books.google.com/ngrams...29&smoothing=3

American English
https://books.google.com/ngrams...28&smoothing=3