Taking complete control to attain rail reforms (12)
I believe the answer is:
totalitarian
'taking complete control' is the definition.
(as in a totalitarian state)
'to attain rail reforms' is the wordplay.
'reforms' indicates anagramming the letters.
'attain'+'rail'='attainrail'
'attainrail' anagrammed gives 'talitarian'.
'to'+'talitarian'='TOTALITARIAN'
(Other definitions for totalitarian that I've seen before include "Despotic" , "One-party" , "Allowing no opposition" , "Of ultimate-State-control system" , "(Of states) extremely dictatorial" .)