Heavy weight transported by our trains once (7)
I believe the answer is:
bouncer
'heavy' is the definition.
(both can mean a member of security staff)
'weight transported by our trains once' is the wordplay.
'weight' becomes 'ounce' (ounce is a kind of weight).
'transported by' indicates putting letters inside (some letters carried or transported by others).
'our trains once' becomes 'br' (British Rail, former British train operator).
'ounce' going into 'br' is 'BOUNCER'.
(Other definitions for bouncer that I've seen before include "trampolinist?" , "Bumper" , "Dud cheque" , "Nightclub door worker" , "Nightclub security doorman" .)