I love trains, but the future is individualized mass transit with autonomous electric vehicles taking you precisely to your destination. Tunnels are needed to address the fundamental choke points in cities. You can’t have 3D buildings and expect a 2D transport system to be effective! Multiple layers of tunnels enable transport to be 3D too. The fastest way to travel long distances city center to city center, short of new physics, is cars/pods in vacuum tunnels.