Insignia of rail age development (7)
I believe the answer is:
regalia
'insignia' is the definition.
(regalia are ceremonial symbols or insignia)
'rail age development' is the wordplay.
'development' is an anagram indicator.
'rail'+'age'='railage'
'railage' with letters rearranged gives 'REGALIA'.
'of' is the link.
(Other definitions for regalia that I've seen before include "Finery" , "Crown, sceptre etc used at a coronation" , "Ceremonial trimmings of high office" , "ceremonial gear" , "Badges of office" .)