Troops finding child on railway (8)
I believe the answer is:
infantry
'troops' is the definition.
(I've seen this before)
'child on railway' is the wordplay.
'child' becomes 'infant' (infant is a kind of child).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'railway' becomes 'ry' (abbreviation for railway).
'infant'+'ry'='INFANTRY'
'finding' is the link.
(Other definitions for infantry that I've seen before include "part of the army" , "Troops" , "Grunts" , "Unmounted fighters" , "fighting group" .)