Easier to sketch it than to explain verbally. Rail corridor and tunnel portal shown in purple with the Blue Line tunnels shown in blue. The K Station PD was pretty good about preserving the corridor for future rail and not putting buildings or deep foundations in the way - K2 preserved a strip of land, Echelon/Alta stair-stepped their podiums around the curved part of the tunnel, and the Jewel just decked over it with a wide span.
West of Halsted is a different set of PDs with different developers, and I'm not sure the city ever preserved that part of the corridor. Sterling Bay started 360 N Green with a design that clearly blocked the corridor, and now they have switched to one where the corridor is effectively preserved with a pocket park so maybe that's a good sign...
I also showed in yellow a separate plan to add a 4th track into Union Station. That is more likely to happen before we get a tunnel, and K Station also preserved some land for that.