to easily annoy someone on a chatroom or on any Instant messaging or social networking site like Facebook, MySpace, etc. by typing "lul" really fast and pressing the return button on the keyboard. you say "LULTRAIN SUCCESS!" if the person gets easily mad or angry. It is referred to the lultrain because you say "lul" like the form of a train going on and on and on.
Victim: UGGGGH STOPPPPPP.
You: LULTRAIN SUCCESS!
(more and more and more until your bored)