Principal's lines are sparkling (6)
I believe the answer is:
starry
'are sparkling' is the definition.
(similar in meaning)
'principal's lines' is the wordplay.
'principal' becomes 'star' (synonyms).
'lines' becomes 'ry' (abbreviation for railway).
'star'+'ry'='STARRY'
(Other definitions for starry that I've seen before include "Stellate" , "Like a clear night sky" , "Like unclouded night" , "Astral" , "Of celebrities" .)