Horrific travel by railway (4)
I believe the answer is:
gory
'horrific' is the definition.
(I've seen this before)
'travel by railway' is the wordplay.
'travel' becomes 'go' (synonyms).
'by' says to put letters next to each other.
'railway' becomes 'ry' (abbreviation for railway).
'go'+'ry'='GORY'
(Other definitions for gory that I've seen before include "Maybe gruesome" , "Involving much blood" , "Covered in blood" , "Involving much bloodshed" , "Bloodstained" .)