Hey Ianrail,
Thanks for the welcome!
The "derelict structures" that I am considering are Pillboxes. They were constructed during the Second World War and several remain intact. These are scatted across London and I have identified 14 that over look the rail system - Hence the specific list of rail lines.
Here is an example of one of the structures:
http://www.skydive.ru/uploads/posts/...7_bridg-55.jpg
Don't worry, I am not planning on trespassing. Still wise to mention it to a newbie though.