The most common and most likely answer to this crossword clue is the 3 plus 4 letter word RAN RIOT
I've seen this crossword clue in The Sunday Telegraph
R
A
N
R
I
O
T
Buddy explains!
The straight part
We know that "went off the rails" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case went off the rails = ran riot.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.