Since Busjack mentioned a maglev I figured I'd throw this idea out:
How about and a route connecting General Mitchell, O'Hare, Midway, Gary, and South Bend airports!
I know it would cost billions and our grandchildren probably wouldn't live to see it...
Not CTA related, but if we really had a transportation policy in this country, we'd invest in a good medium-to-high-speed railroad linking O'Hare with the Midwest region, with frequent service on dedicated tracks. Some trains would run to downtown Chicago, serving regional travel needs, and some would serve O'Hare, taking care of longer-distance (and international) connectivity. Depending on the route, some could perhaps serve both, during off-peak times.
Milwaukee/Mitchell would be a good secondary airport, and Midway could probably hold its own for traffic.
There's really no point in having one rail line connect all of the above airports, in particular Gary and South Bend. There's no conceivable scenario where one would be scheduled to fly into South Bend with a connecting flight from O'Hare or Midway. The actual traffic is generally to/from the cities, not the airports, so it's more important to connect an airport hub with a bunch of city centers.
A good regional rail system would reduce (or in some cases, eliminate) the need for short-distance flights out of O'Hare which only serve as feeders to long-haul flights. If trains could serve those markets instead, that would free up capacity at O'Hare that would possibly eliminate the need for relief airports in the Chicago region.
Take a look at how the TGV transformed air travel in and around France over the last three decades as a model for how this could work.
Of course, to tie this in to a relevant transit topic, one way that would be helpful in having this sort of project succeed would be good transit connections at the destinations. Chicago, relatively speaking, actually has pretty good transit, but other cities such as Indianapolis, Detroit, Toledo, etc. really leave a lot to be desired when it comes to transit connectivity.