For the episode "Union Station", in order to keep Sherry Stringfield's departure a secret, false copies of the script circulated amongst the production staff. Only writer Carol Flint, director Tom Moore, the shooting crew, Stringfield, and Anthony Edwards knew the ending ahead of time, and it was filmed on location in Chicago the Saturday before the episode was slated to air. Editor Kevin Casey got a call the following Monday from a confused assistant saying that the footage of the final scene which had just arrived did not match the script. Flint eventually set them both straight and Casey went to work, editing right up to the airdate. NBC didn't even know what was actually going to happen before the episode aired.
At this stage in the series, George Clooney would be working 4 days on "ER" and 3 days on Batman & Robin (1997). This meant that he was invariably extremely tired on set.
Susan's apartment and building is a real apartment block in Chicago. The scene where Mark goes there to look for her towards the end of the episode was filmed at The Hale Loft building located at 14 North Peoria Street in Chicago. The building is still there unchanged to this day.
The 'L' Train station that Mark waits at towards the end of the episode is a real station. The scene was filmed on location at Quincy station located on South Wells Street in Chicago.