Is digging down really cheaper than going up? Chicago has got an elevated train [0] and even double decker streets [1] and those don't have lots of the problem

Is digging down really cheaper than going up? Chicago has got an elevated train [0] and even double decker streets [1] and those don't have lots of the problems that tunnels do like ventilation, escape tunnels, and moving all the dirt. [0]: https://en.wikipedia.org/wiki/Chicago_%22L%22 https://en.wikipedia.org/wiki/Chicago_%22L%22 [1]: https://en.wikipedia.org/wiki/Wacker_Drive https://en.wikipedia.org/wiki/Wacker_Drive