Characters aboard train in Euston Square (4)

I believe the answer is:

nine

'square' is the definition.
(I've seen this before)

'characters aboard train in euston' is the wordplay.
'characters' indicates the answer is hidden within the clue (some letters or characters from the clue).
'NINE' is hidden within 'aboard train in euston'.

(Other definitions for nine that I've seen before include "One over the eight" , "Number of Muses and days in a Novena" , "Short time for a wonder" , "Days Lady Jane 5 ac reigned" , "SECOND PART OF 14" .)

I've seen this clue in The Telegraph.

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