There's more. First of all, you can only ever put a max of 3 steam points into any function, which means d6 + 3d6 max. Secondly, you only do impact hits in close combat on the turn you charge. In subsequent rounds you grind in the movement phase. You could still win a combat with the steam gun, the Engineer's attack and/or a flank bonus but as mentioned, a Tank cannot pursue or overrun.