Joliet is a great choice, and you can take METRA (the local operator of commuter trains) to get there. I believe they have recently finished a railfan pavilion which would be a great location for watching or photography (I believe you will be on the east side of the tracks, so try mornings).
The other classic place with a pavillion is Rochelle, but you need an auto to get there. In terms of geography you will be on the south side of the UP tracks, and the northeast side of the BNSF.
If you are downtown and appreciate buildings, be sure to take in Union Station. The portion that is the access to the platforms is the typical commuter rabbit warren, but find a station diagram and go to the portion they call the Great Hall.
There are many other good locations in the Chicago area, but I hesitate to recommend them to a foreigner who may not recognize signs of our more dangerous neighborhoods.
And, by the way, welcome to the US!