Worthless loco finally retired from sweet old railway (8)
I believe the answer is:
nugatory
'worthless' is the definition.
(similar in meaning)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for nugatory that I've seen before include "Triflingly unimportant" , "Futile, of no value" , "Insignificant or trivial" , "useless" , "Young art (anag) -- worthless" .)