-  Travel and travail  are doublets –
that is to say, they have a common ancestor, but
have split into separate words. Their ultimate
source is medieval Latin trepālium, a term for an
instrument of torture made of three sharp stakes.
This was a compound noun formed from Latin
trēs ‘three’ and pālus ‘stake’ (source of English
pale). From it was formed a verb *trepāliāre
‘torture on the trepālium’, hence generally
‘torture’. This passed into Old French as
travailler, where its reflexive use ‘put oneself to
pain or trouble’ evolved to ‘work hard’. Its noun
derivative travail ‘painful effort, hard work’ was
borrowed by English as travail, and this quickly
developed a new sense, ‘journey’ (presumably
from the notion of a ‘wearisome journey’),
which came to be distinguished by the spelling
=> PALE, THREE, TRAVAIL
* * *The verb meaning 'journey' developed from the verb travail, meaning 'labor,' the concept being that a journey is a laborious business, one requiring effort and possibly discomfort. Travail itself derives from Old French travaillier (compare modern French travailler, 'to work'), in turn from a conjectured Vulgar Latin verb tripaliare, 'to torture,' formed from the Late Latin noun trepalium. This was an instrument of torture constructed from three stakes, and itself represents a blend of Latin tres, 'three,' and palus, 'stake.'
The Hutchinson dictionary of word origins. 2013.