Originally Posted by
HeHateY
If you are already at Union Station, why not just take METRA's North Central Line to O'Hare Transfer.
Schedules here:
http://www.metrarail.com/Sched/ncs/ncs_wkout.shtml
Fare is US$3.80 one way, half that for Seniors.
At the O'Hare Transfer station, a bus (low floor, IIRC) will meet you and take you to the end (Remote Parking Station) of the Inter-Terminal people mover called "ATS" (or as I like to call it, O'Hare-VAL):
http://en.wikipedia.org/wiki/Airport_Transit_System
Then ride that to your desired terminal.
Cheap and relatively painless!
HeHateY: I think you answer your own question later in the post. Taking a Metra train to a bus to another train is enough of an issue that many previous posters have advised against this option. Too many failure points, it seems like to me.