Loads more from the London Underground. When travelling on a Metropolitan line train between Moor Park and Croxley, at the beginning of the Watford South Curve where you branch away from the Amersham/Chesham tracks, on the right hand side a large empty space was used to be part of "Croxley Tip" - a landfill site made for dumping waste whilst demolishing the Neasden Power Plant until the mid-1980s. The ex-GWR Pannier Tank would normally travel there (requires running around at Watford because it can only be entered southwards), before being replaced by the battery-electric locos.
Here's a map where Croxley Tip was:
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The entrance is visible from a nearby bridge.
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A couple more from London Underground:
~ The 1890 C&SLR depot incline at Stockwell. Part of this survives, hidden away in a network of what are now ventilation and cable tunnels. Nearby is the disused station tunnel cavern, with some old tiling visible in places if one knows where to look.
~ Weston Street signal cabin, between the tunnels mid-way between King's Cross and Angel
~ The Embankment loop, most of which survives intact, although over half of it lying beneath the river is permanently sealed off. The remaining accessible section has some rather fine stalagtites.
All these features are very much hidden from view!
For those into impressive structures, some of the 1960s "cathedral" crossover caverns on the Victoria Line are also slightly awe-inspiring, along with some of the tunnel works associated with mundane structures like ventilation shafts and the like. Ironically the more modern lines can be more impressive in this respect, the Jubilee has some impressive features too, both on the 1970s section and the JLE, as well as the Heathrow part of the Picc Line.