Good distance south of Slough, comprehensive training area? (11,4)

'training area?' is the definition.
(area for separating trains onto different tracks)

'good distance south of slough comprehensive' is the wordplay.
'good' becomes 'g' (abbreviation).
'distance' becomes 'yard' (I've seen this before).
'south of' says to put letters next to each other.
'slough' becomes 'marsh' (both can mean a swampy area).
'comprehensive' becomes 'all-in' (all-in can mean covering everything).
'g'+'yard'='gyard'
'marsh'+'allin'='marshallin'
'gyard' after 'marshallin' is 'MARSHALLING YARD'.