Bennerley Viaduct is one of the few iconic railway structures still standing in my area. All the brick built viaducts bridges etc were demolished and the rubble used beneath by-passes in some cases. The Bennerley Viaduct owes it's existence as a grade 2 listed building purely because it wasn't worth knocking down, it is built from wrought iron that can't be cut with oxygen acetylene flames. They would have had to dismantle it bit by bit at enormous cost in labour.
It is soon to have another use as a cycle track and footpath, how you will get up there on a bike remains to be seen.