This article is from the ER FAQ, by Rose Cooper firstname.lastname@example.org with numerous contributions by others.
Always good friends, it looked as if Mark and Susan were
finally going to get together romantically during the third season
thus fulfilling a lot of viewers` wishes.
This story turned into heartbreak for poor Mark, however, when
Susan announced that she was leaving for Phoenix to be with her
sister Chloe and family. In the episode "Union Station" , Susan
made good on this ambition, and left on a train never to be seen
Naturally, this means that Mark and Susan will not be
getting together after all.
[...so, why does the phrase "famous last words" keep coming to mind?