Thinking more into the idea of the upgraded tunnels, you could also have a public transport service building that these upgraded tunnels are used for. this building consumes power and can be upgraded to the various tiers to produce more efficient transportation across longer distances. My first idea would be either some internal tram that works its way around or perhaps a skybridge or buried tunnel system for seperate transportation to smaller access stations you have to build where citizens utilizing the transport system can enter and exit.