Providing you started the tunnel system in a remote area you could run all the dirt out one exit, so it would not necessarily mean that if there tunnels in D.C. or elsewhere the dirt could be coming out any of the access ways. Unlike something like the euro channel tunnel between London and Paris where people started on either side so the only place to put the dirt was out each sides respective entrance. If only one boring machine started in London then all the dirt would be in London