A crew can drive on a single heading an eight foot by six foot tunnel through competant sediments easily enough at a pace past thirty feet per day. Actually a lot faster but that is still two to three miles per year. More likely it will approach ten miles per year.
Manpower would likely be a company of 200 men for a two shift operation. Thus ten crews can walk out a hundred miles per year. Ten years gives us a thousand miles. what you can easi8ly see is that it becomes completely possible to produce an extended network wherever it is important and even to contemplate long distance connections as well. All tunnels would be tracked and likely use electric rail cars for the most part.
I knew of this system from other sources, but also had insider confirmation on the reality of such military tunnels. It was always one of the best responses to the Nuclear threat. It is also cheap when you already have manpower needing to be tasked..
Retasking it all to support a criminal enterprise is a natural decision.