There are no trains in Realm of the Mad God - well, not the kind that go along tracks anyway. Yet rarely a minute passes without some reference in the chat window that every player can see. "Train anywhere?" "Choo Choo?" "Where train pls?" And then comes the glorious moment when a soul stops to type "TRAIN HERE," and all the cool kids teleport to their location.
A train in Realm of the Mad God is a big bunch of players that find a road, and then follow that road on its never-ending loop around the world. Auto-fire is turned on, everyone's moving at a slightly different speed, and every single enemy that's in front or to the side of this force of nature is obliterated in an instant.
A train is not, strictly speaking, part of Mad God's rules. It exploits them, slotting perfectly into the system as well as being tremendous fun. Killing enemies as part of a group rewards each member of that group with XP - and this figure is not divided, so every player involved gets the full amount. So players form trains to level up at an extraordinarily fast rate. The game can barely handle it at times, especially when you hit a gigantic gaggle of beasties that are obliterated in seconds, the frame-rate slowing to a crawl as multitudes of green numbers and 'level up!' notices fill the screen.