The most common and most likely answer to this crossword clue is the 11 letter word DISCIPLINES
I've seen this crossword clue in The Guardian
D
I
S
C
I
P
L
I
N
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 = disciplines.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 DISCIPLINES mean?
DISCIPLINES noun- A controlled behaviour; self-control.
- A specific branch of knowledge or learning.
- To train someone by instruction and practice.
- To teach someone to obey authority.
- To punish someone in order to (re)gain control.
- To impose order on someone.