A term referring to the rotating governors used on steam locomotives and related steam engines such as tractors. The brass balls acted as weights on the end of linkages, and rotated with the increase in RPM of the engine. As speeds rose, the balls swung outwards, rising on the linkages. At a pre-set height, the release valve would engage, lowering steam pressure and reining in the RPM to the allowable maximum. The balls rose towards the firewall and/or the walls of the cab., hence the term.
The Union Pacific Freight roared across the Kansas night, balls to the wall.