Comment Re:Expensive (Score 1) 117
Bored railway tunnels are only single-track, usually with two parallel bores. Here are some good photos, I believe the "cathedral"-sized cavern was built by digging down from ground level. The finished tunnel diameter is 6.1m.
The London Post Office Railway has 2.7m tunnels, so is pretty much what you want. It was shut down after the introduction of the Congestion Charge, since that removed enough traffic that it was then cheaper to use surface vehicles.