Originally Posted by fairviewroad

The OP states her father is due in to Union Station around 3 p.m on a weekday.

Metra trains depart Union Station to O'Hare at 4:25, 4:58 (express), 5:01, 5:31, and 6:00. So even with a late arrival into Union Station, this would be a suitable choice. The 6:00 p.m. train might be cutting it a little close if the passenger has 5 hours to make the connection (presumably this means an 8 p.m. flight).