[Skye]

This concept is not new at all, From what I understand the tunnel sizes proposed are barely bigger than the London Underground "tube" tunnels. (Glasgow's subway has even smaller tunnels too). Now, don't get me wrong, it worked quite well for London, but it's not exactly innovative.

[geezerjay]

It should also be noted that london's postal service also deployed 2ft-wide tunnels to cart mail through the city, which was launched in the late 1920s.

https://en.m.wikipedia.org/wiki/London_Post_Office_Railway