Mechanical Wagon Piece

The Destroyer drops this one, and only into a treasure bag, which means an Expert or Master world. It is guaranteed, so there is no rate to grind and no reason to kill the worm a second time for it.

The catch is that all three mechanical bosses have to fall before the parts add up to anything. Until the last one arrives the piece sits in your inventory doing nothing, so put it in a chest rather than carrying it around.

Nothing here is time sensitive and none of it is a power spike. The cart it eventually feeds is a convenience for a world where you have already built rail, so slot the mech bosses in the order you wanted anyway and pick the parts up as they come.