The issue with tunnels has never been the tunnel itself but the cost of all the supporting infrastructure. You need regular access shafts both for emergency use and for normal egress. And the normal access shafts (subway stations/highway ramps) must be very large to accommodate reasonable throughput; Elon Musk wants to solve this with car elevators, but both cars and elevators are low capacity. You also need large ventilation structures even with electric propulsion because modern fire codes require the quick exhaust of smoke from a fire. And all this tunnel infrastructure has fairly large surface requirement, at least but not only during the time of construction, so in an urban area property acquisition is a significant chunk of the budget.