The most common and most likely answer to this crossword clue is the 6 plus 6 letter word MAKING TRACKS
I've seen this crossword clue in The Times
M
A
K
I
N
G
T
R
A
C
K
S
More clues leading to the result MAKING TRACKS
Buddy explains!
The straight part
We know that "Leaving/the job for rail builders to do" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case leaving/the job for rail builders to do = making tracks.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.