From O'Hare Airport to Amtrak Union Station
All Amtrak trains serving Chicago depart from Union Station. The best way to travel between the airport and the Amtrak station is by the CTA rail. Trains are fast and run frequently, and the entire journey (including all waiting and walking) takes approximately 1 hour 10 minutes. Beware that private shuttles run much less frequently than the CTA trains do (about once an hour compared to every 5-10 minutes), and so on average you are not going to save much time by taking a shuttle despite paying at least 10 times as much as the cost of a train ride. Moreover, trains run at all times, and trains do not risk getting stuck in Chicago traffic. Unless you have a lot of heavy luggage, a train ride is a more sensible option in most cases. The two maps you will need for this journey are the CTA rail map and the Downtown Chicago map.
First of all, you need to get to the CTA station (O'Hare station on the Blue line). If your flight arrived at any of the domestic terminals (1, 2, or 3), just follow the signs "Trains to City" from any terminal. The walk from any gate to the station takes about 10 minutes. If your flight arrived at the International terminal (5), you will need to take the Airport Transit System (ATS) first. Follow the signs "Trains to City" or "Airport Transit System" and get to the ATS station. Take an automated train to Terminal 3, get off, take an escalator up, turn left, walk to the elevators in the parking building, take an elevator down to the "CTA" level, and follow the underpass to the CTA station. The entire trip takes about 15 minutes.
Now you are at the CTA station. The fare is $2.25. Buy a transit card from the farecard machines (cash only). Exact fare is required; however, one transit card can be shared by several people - just put enough money for all the riders. Pay your fare and pass through the turnstiles to the boarding platform. Take any train and travel to Clinton station on the same Blue line. Travel time from O'Hare to Clinton is 46 minutes; trains run every 7-8 minutes on weekdays and Saturdays and every 10 minutes on Sundays and holidays.
Finally, you need to get from the CTA station to Union Station. Union Station is located two blocks (300 m) north of Clinton station (map). Simply walk north along Clinton (there is a sign pointing in the direction of Union Station at the exit from Clinton station). You will see the entrance to Union Station near the intersection with Jackson. The entire walk takes about 5 minutes.
The total travel time from O'Hare to Union Station should be about 1 hour 10 minutes including all walking, waiting and riding the train (about 1 hour 15 minutes if you are coming from the International terminal). So, it is safe to have 1 hour 20 minutes to make this connection between the airport and the Amtrak station.
The Blue line operates at all times, and so there is no last train to miss.
If you still have any questions, feel free to email me (no phone calls please).