I know Central has the ghost platforms 26 and 27 that have never seen a train.
If you go down the stairs from the concourse toward platforms 24/25 you will notice a door at the level of 26/27.
Not for public access though.
I did find some good pics taken by David Johnson
HIS WEB PAGE
Platform 27 and 26

Platform 27

Both platforms have very short stub tunnels ready to be extended if ever the platform is to be used.
Platform 27 stub

Platform 26 stub

I wonder if it would be possible to get access for even 30 min
I know there are far better tunnels in ST James but as far as i know a full no go zone unless you want to risk huge fines as you need to get to then via the active subway. Also very dangerous.
Be nice if there was a safe and easy way to see them