From Diablo Wiki
Train can refer to training, the process of spending gold and materials to upgrade an Artisan's level.
It's also used as an in-game term for a long row of monsters, usually chasing after one character who is leading the monsters into a kill zone. This tactic is common in MMOs like World of Warcraft, but isn't often seen in the Diablo games, where party mechanics are not so rigidly defined.
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