Train direction must be changed mechanically. The engineer pushes or pulls a lever (called the Johnson bar) in the cab to accomplish this change. Some locomotives have a steam-operated power reverse cylinder which reduces the physical effort required of the engineer. Valve gear consists of a radius arm, reversing shaft arm, reverse (or combination) link, and link bearing (or link support). The reverse link pivots up or down, moved by the radius rod on the bearing, which rocks back or forward to determine the locomotive's direction.