When someone takes his/her leave during a chat session or in an active forum thread people tend to say goodbye which results in either a topicchange or the end of all conversation in general.
A: Pollution, yeah....
B: I've to go now, bye everyone!
C: Bye!
D: Bye!

10 minutes later....

A: Damn, bye train killed the chat
by Sjieskebab September 14, 2010
Get the bye train mug.