Both Uses

serpent
in
The Railway Children
(Edited)

It's quite right what it says in the poetry-book about sharper than a serpent it is to have a toothless child — but it means ungrateful when it says toothless.
p. 50.9 *

serpent = snake
He told them many things that they had not known before — as, for instance, that the things that hook carriages together are called couplings, and that the pipes like great serpents that hang over the couplings are meant to stop the train with.
p. 51.4

serpents = snakes

Definitions:

(1) (serpent)
a snake
(2) (meaning too rare to warrant focus)
For less common senses of "serpent", see a comprehensive dictionary.