The problem with using the Blue Line as the basis for an airport express is that the old Metropolitan/Logan Square L is two-track-only and exceedingly hemmed in by buildings on both sides. In particular, the buildings that back onto one side of the L front onto busy Milwaukee Avenue, so (1) the buildings are worth more, all else being equal, and (2) tearing them down would turn a relatively vibrant main street into somewhat of a a wasteland. The tunnels of the "new" (late '60s) Kimball subway and the '40s-'50s-vintage Dearborn-Milwaukee subway are two-track-only so that third-tracking or passing sidings would have to be in essentially new subway, with all that implies about expense. The only place with room to build a third track or passing sidings without great expense would be where the Blue Line runs in the Kennedy Expressway median, which is only the northern/western half (roughly) of the O'Hare-downtown portion of the Blue Line; in other words, absent the aforementioned expensive and disruptive L/subway construction, airport expresses would have to do all their third-tracking or passing northwest of Belmont.
This is basically the issue. Plus, in order to do express service properly, you'd need four tracks (not three), and they'd have to run the full length, not just "passing sidings" every here and there.
The Blue Line is already a busy railroad (second busiest line in the CTA system), and throwing in the complexity of switching tracks on a service that already runs a peak headway of about four minutes would destroy the reliability of both the existing service and the express service.
Three tracks would only be good for directional express running, meaning the reverse direction would still have to operate local (which, during rush hour, could very well overload the railroad).
Essentially, you'd need a new right of way to be able to fit four tracks, and you're not going to find that above ground in too many parts of Chicago.
The Metra idea would work better, but still face a few challenges. First would be track capacity at Union Station, particularly during rush hour. Second would be capacity on the railroad between Union Station and A5 (where the Milwaukee West/North Central service splits off from the Milwaukee North/Amtrak service).
As I recall, the running time between Union Station and O'Hare, if run nonstop, is about 25-30 minutes. This would have to be sped up somehow, though I don't know how it could. I don't know how long it takes to ride the O'Hare people mover to the point closest to the Metra station now (I've only ridden that thing once or twice ever, and that was to/from the international terminal, not remote parking), but once you factor in the time to wait for the people mover, the running time to the Metra station, and the time to wait for the Metra Airport Express, you're approaching the 45 minute running time of the Blue Line from O'Hare to Clark/Lake for its existing service. A Metra Airport Express would really only be more convenient (vs. the existing Blue Line) for those going to places west of the river (Union Station/Greektown area). Otherwise, the Blue Line, despite being more crowded during peak periods, is going to get more people closer to their final destinations.
If money were no object, a better answer would be to build a rail station connected to the O'Hare terminal (like the Blue Line is), and a subway connector that actually serves Union Station and circulates around the loop. That way, you'd get a fast, low-hassle ride to downtown, and an easy enough connection to the rest of downtown.
