The most common and most likely answer to this crossword clue is the 4 plus 5 letter word BOAT TRAIN
I've seen this crossword clue in The Times
B
O
A
T
T
R
A
I
N
More clues leading to the result BOAT TRAIN
Buddy explains!

The straight part
We know that "coast-bound transport" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case coast-bound transport = boat train.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.