The underground system exists
And it's existed for about 150 years during which time just about every economically feasible mitigation for its capacity constraints has been implemented.
The frequency of the trains is constrained by their ability to brake safely, the length of the trains is constrained by the size of the stations, the size of the stations is constrained by the ability to get people in and out through the corridors and escalators.
Putting hyperloop technology into existing tunnels can do nothing other than reduce their capacity...
Biting the hand that feeds IT
