Man on train's an old friend (12)

I believe the answer is:

schoolfellow

'an old friend' is the definition.
(friend from school)

'man on train's' is the wordplay.
'man' becomes 'fellow' (fellow is a kind of man).
'on' says to put letters next to each other.
'train' becomes 'school' (both can mean to teach or coach).
'fellow' after 'school' is 'SCHOOLFELLOW'.

I've seen this clue in The Guardian.

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