Descent or ascent arranged by railway (8)
I believe the answer is:
ancestry
'descent' is the definition.
('ancestry' can be a synonym of 'descent')
'ascent arranged by railway' is the wordplay.
'arranged' is an anagram indicator.
'by' says to put letters next to each other.
'railway' becomes 'ry' (abbreviation for railway).
'ascent' anagrammed gives 'ancest'.
'ancest'+'ry'='ANCESTRY'
'or' acts as a link.
(Other definitions for ancestry that I've seen before include "Genealogy" , "Person's ethnic descent" , "Line of forebears" , "race" , "Blood line" .)