These pictures show what can be involved in re-doubling a railway. It puzzles me slightly as to the extent of the heavy engineering, as the line was originally double track. So quite why cuttings need widening and bridges demolishing I'm not sure (suppose electrification may be the reason for the bridges).
http://www.airdriebathgateraillink.co.uk/project/gallery/It's possibly not surprising that heavy engineering work is needed especialy in cuttings. The Old Worse and Worse was a bit of fly by night concern and the line was not over engineered, certainly not to Brunel standards. Probably what's happened since singling is that the cutting sides further from the tack have been allowed to consolidate (slip) and encroach onto the old trackbed.
Chiltern found similar problems when doubling from Risborough to Anhyo particularly in cuttings even though that line was of much later construction.
I can't however offer an explanation for the need to rebuild bridges except that the permanent way is the least permanent part of the railway.
