Unfortunate inertia over railway travel plan (9)
I believe the answer is:
itinerary
'travel plan' is the definition.
(synonyms)
'unfortunate inertia over railway' is the wordplay.
'unfortunate' indicates an anagram.
'over' means one lot of letters go next to another.
'railway' becomes 'ry' (abbreviation for railway).
'inertia' anagrammed gives 'itinera'.
'itinera'+'ry'='ITINERARY'
(Other definitions for itinerary that I've seen before include "Plan of a journey, route" , "Proposed route of travel" , "Planned route" , "Travel plan" , "List of places" .)