Control a section of Hertfordshire's train tracks (9)
I believe the answer is:
restraint
'control' is the definition.
(I know that control can be written as restraint)
'a section of hertfordshire's train tracks' is the wordplay.
'a section of' indicates the answer is hidden within the clue.
'RESTRAINT' is hidden within 'hertfordshires train tracks'.
(Other definitions for restraint that I've seen before include "Restriction, limit" , "Moderation" , "It keeps someone or something under control" , "Self-control" , "Curb" .)