Rigid principles for the running of public transport (9)
I believe the answer is:
tramlines
I believe this clue is a double definition.
'rigid principles' is the first definition.
(I've seen this before)
'for the running of public transport' is the second definition.
(trams run along tramlines)
(Other definitions for tramlines that I've seen before include "rigid principles" , "Tracks" , "Side markings of tennis court" .)