Travelling by train out - isn’t flying (2,7)
I believe the answer is:
in transit
'travelling' is the definition.
'train out isn't flying' is the wordplay.
'out' indicates an anagram (out can mean wrong or inaccurate).
'flying' is an anagram indicator (letters fly around into new positions).
'train' anagrammed gives 'intra'.
'isnt' with letters rearranged gives 'nsit'.
'intra'+'nsit'='IN TRANSIT'
'by' is the link.
(Other definitions for in transit that I've seen before include "while travelling" , "En route" , "on the move?" , "(Of goods) being moved" , "on the wagon?" .)