If I'm not mistaken, I think you'd actually have to take the train back downtown to Union Station and then back out to Arlington Park on the Metra because all of the lines that go to the suburbs don't run into each other but are all joined at Union Station.

that's absolutely correct. to get to AP from O'Hare by train, you'd have to take the blue line (subway) downtown to Union Station, then transfer to a Metra train and go back out to the suburbs.... way too much of a pain to do.

No, you don't have to do that. You can take the "L Blue Line" from O'Hare to Jefferson Park and jump off there, and catch the Metra there to Arlington Park

Good call -- still just seems like a lot of work when the airport is really so close to the park already.

It is, but it is possible for someone that doesn't want a rentacar or can't get one.....