Boiler and stationary engine on the back wall, and all that lovely lineshafting!  Lineshafts were the preferred method of power transmission in the steam days. They were versatile (even included variable drive speeds by switching a belt from on drive pulley to another), could accommodate different rotational directions, and were simple and easy to maintain. Those darling of the steampunk imagination , gears, were not generally used as a source of major power transmission, too costly to build and maintain, in a large stationary power plant setting, such as a mill.  Gears did find their way into steam traction engines, as the primary way of transferring power to the tractor wheels, and there were some specialized  geared steam locomotives (Shay, Climax, Heisler and Hunslet come to mind).