When there is a group of people and when one person leaves a number of people tend to leave right behind them. One after another much like a train.
At a party once one person leaves, people tend to leave right after them.

-or-

When playing video games and the game is going very poorly for one team, once one person on the team leaves, the rest tend to follow shortly after.

"There goes the LeaveTrain"
by AlexDGreat January 11, 2013
Get the LeaveTrain mug.