So here is the proposal for the underground tunnels from the RAND corporation. Rather then use them for public transport the military instead created them to connect the underground bases they have built, using very high speed magnetic levitation trains. This is from 1972. The tunnels exist.
https://www.rand.org/content/dam/rand/pu.../P4874.pdf
https://www.rand.org/content/dam/rand/pu.../P4874.pdf