Terrain to have an effect on railway (7)
I believe the answer is:
country
'terrain' is the definition.
(thesaurus)
'an effect on railway' is the wordplay.
'an effect on' becomes 'count' (I can't explain this - if you can you should believe this answer much more).
'railway' becomes 'ry' (abbreviation for railway).
'count'+'ry'='COUNTRY'
'to have' is the link.
Can you help me to learn more?
(Other definitions for country that I've seen before include "Nation" , "See 9" , "land?" , "State; region" , "Rural" .)