A quick google search shows that Chicago is already spending .5bn on improving the blue line (https://www.transitchicago.com/yournewblue/).

Why would they approve this project if they didn't feel this was sufficient?

Only thing I can think is that the demand is going to be strong or they're thinking it's a steal with the contractor buying it out and are willing to experiment.

I'm all for new transit solutions and I'm really interested in this one, picking this route in particular though is what's odd to me...