Look into head of public railway (3)
I believe the answer is:
pry
'look into' is the definition.
(I've seen this before)
'head of public railway' is the wordplay.
'head of' indicates taking the first letters.
'railway' becomes 'ry' (abbreviation for railway).
The initial letter of 'public' is 'p'.
'p'+'ry'='PRY'
(Other definitions for pry that I've seen before include "Look inquisitively into private matters" , "Intrusively enquire" , "Be nosey" , "Enquire too inquisitively" , "Be inquisitive" .)