It's a 'Salmon' with a roof. Salmon are flats for carrying track panels or concrete sleepers, this is what most that are left look like: They now have new bogies and most don't have anything on the deck other than possibly some bolsters. This one here has been sat just round the corner from where I live for well over 5 years now, this one still sports the old plate-frame bogies and cranes/generator for unloading - those are features that yours also has... Some of these that were based in electrified regions got the 'roofs' to prevent workers from getting too high up and touching the live wires - this is a shot of one more like yours, from the site of Mike Cubberley, an old mate of mine. http://mike6874.fotopic.net/p25541051.html