The most common and most likely answer to this crossword clue is the 7 letter word TEACHES
I've seen this crossword clue in The Daily Telegraph
T
E
A
C
H
E
S
Buddy explains!

The straight part
We know that "trains" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case trains = teaches.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.
What does TEACHES mean?
TEACHES verb- To show (someone) the way; to guide, conduct; to point, indicate.
- (ditransitive) To pass on knowledge to.
- To pass on knowledge, especially as one's profession; to act as a teacher.
- (ditransitive) To cause to learn or understand.
- (ditransitive) To cause to know the disagreeable consequences of some action.
- (usually as a term of address) teacher
- One of the series of boilers in which the cane juice is treated in making sugar; especially, the last boiler of the series.