'Fraid not Dassie. Just so you know I used to be a Civil Engineer building roads etc and am now a truck driver (long story!) and my hobby is Industrial Archaeology. The vast majority of track in this country was single or twin. Only major lines are 4 track, and of those I don't know any that have been reduced to twin track. Although some trackbeds are still in existence, they are usually no where near complete. In urban areas they have been built on and in rural areas embankments have been levelled and cuttings filled in to make better use of the farmland. Another example; most of the old Great Central viaducts in the Midlands have been demolished for their blue engineering bricks. The lines could theoretically be rebuilt, but it isn't going to happen unless you want the Government to close down the NHS, Welfare State, Schools etc 'cos even that sort of budget wouldn't scratch the surface.