Used rail transport and left (8)

I believe the answer is:

residual

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'left' is the definition.
Although both the answer and definition are adjectives, I cannot understand how they can define each other.

'used rail transport' is the wordplay.
'transport' indicates anagramming the letters (I've seen 'transported' mean this).
'used'+'rail'='usedrail'
'usedrail' is an anagram of 'RESIDUAL'.

'and' is the link.

Can you help me to learn more?

(Other definitions for residual that I've seen before include "Leftover - used rail (anag)" , "unused" , "Pertaining to what is left over" , "Left over as a remainder" , "surplus" .)

I've seen this clue in The Globe and Mail.

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