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A "train" in EQ1 was where aggroed mobs might go after folks outside your group.
Ex. say you are in a group that takes on the Uber Ogre. Things go wrong and half your group gets wiped out, so the remaining half scatter and run, every man for himself, because the penalty for dying has some sting to it (hopefully).
So, in running, you dart past some other players who are talking or resting/medding or haggling over the price of something, and they are caught totally by surprise. The Uber Ogre, depending upon how its aggro is still on you, peels off and attacks those players, while you continue to run to safety. And the player who had stepped away getting a cup of coffee while his mage medded, might come running back to the keyboard, stumbling, fumbling, spilling, cursing and scrambling to get his character back into action.
Now, "trains" were a minor form of griefing, intentional or not, and certainly require some sort of faction penalties or, in extreme cases (constant griefing), GM intervention. But, "trains" were immersive and kept you on you toes. Noobs in the area can get you killed, so entering any dangerous area requires a visual scan of who is about, are there corpses littered on the ground, are folks griping in chat, etc....
This game mechanic kind of kept things lively and served to remind you that you were truly in a shared world. Heck, depending upon where you "camped", you might even log into some of this drama.
BUT.... will EQ Next have trains? I don't know.