Restricts studies on railway traffic (10)

I believe the answer is:

constrains

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'restricts' is the definition.
(constraining is a kind of restricting)

'studies on railway traffic' is the wordplay.
'studies' becomes 'cons' (to con is to study or revise).
'on railway traffic' becomes 'trains' (I am not sure about this - if you are sure you should believe this answer much more).
'cons'+'trains'='CONSTRAINS'

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I've seen this clue in The Globe and Mail.

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