Simple carbon, for example, used on a railway (10)
I believe the answer is:
elementary
'simple' is the definition.
('elementary' can be a synonym of 'simple')
'carbon for example used on a railway' is the wordplay.
'carbon for example' becomes 'element' (carbon is a kind of element).
'used on' is a charade indicator (letters next to each other).
'railway' becomes 'ry' (abbreviation for railway).
'element'+'a'+'ry'='ELEMENTARY'
(Other definitions for elementary that I've seen before include "description of subatomic particle" , "schooling?" , "One precursor to 7 [SECONDARY MODERN]" , "Easy" , "Principal" .)