The fireboxes of many United States locomotives (though not the K-28s) are too large for firemen to adequately hand-shovel, so they use a mechanical stoker to crush the coal and force it into the firebox. As grates grew too large to fit between the rear drivers, fireboxes had to be pushed backwards to rest on trailing trucks (which grew from two to four and finally six wheels to support the massive grates and fireboxes).