Why does it cost $5 billion to upgrade? The article says they have a working, reasonably modern system.

So is it $5 billion worth of excavations or construction? How much does it really cost to add these cables? Couldn't the cables go in metal tubes attached to the walls of the tunnels? How does that cost $5 billion?

This Geoff Mashall video has some signalling systems from the 1880s and ends on some up to date ones.

https://youtu.be/UCXav-ptPeA?si=UzzyU8FSV11a67fN