If Chicago has the same underpinning of clay much of London has and he's below basement level, dig cost can be pretty tractable. That said, I too have doubts the model can work.

Basically, I think he's overstated the potential savings from process improvement and understated the on costs of safety and compliance to standards like air quality and escape paths.

Chicagoan here. What the city really needs is a ring line - a wheel for all the spokes of current train lines. There's already a CTA train going from O'Hare to downtown (and back out again!). It's hard to imagine this having much appeal besides a shiny new toy for the consultants flying into the city.