A closer look at the adding of the ballast, gives a better idea of what's going on. Here we see the load of steel from the rail cars being added as counterweight. Note the row of round holes along the left edge of the picture, these are the fill holes for the ballast compartments. ( If you look closely, you can see one just in front of the workers right foot). The scrap steel is dumped ( and shoveled ) into these holes until the compartments below are full, then a round steel plate is set into the hole and welded in place to close the hole.