They built a tunnel just for mail trains between postal offices in London in the 19th century. You can even go down there now https://www.postalmuseum.org/event

They built a tunnel just for mail trains between postal offices in London in the 19th century. You can even go down there now https://www.postalmuseum.org/event/tunnel-walks/ https://www.postalmuseum.org/event/tunnel-walks/ I'm... not convinced it scales well, cost-wise!