The tubey bit marked with a question mark in the photo isn't a kit part but the brass hinge that allows it to tip up. Dave's got his hinge a little too far back and effectively created a second dummy hinge. It's stuck to the underside of the waffle pattern Airfix girder bridge base that spaces apart the girderwork sides. Only then should the additional bridge bases be added inside to brace the girderwork sides.
Amazing, now you've said that it's cyrstal clear! Couldn't work that out at the time. Oh well...
On the subject of identifying parts, nobody was ever able to identify what toy the side tracks came off (actually on the drill part, not the tractor)