The Walschaerts Gear is characterized by being fitted completely on the Outside of
the driving wheels. The 90° offset is obtained from a Sub Crank mounted on the
out side of the main driving wheel Crank Pin. The length and angle of this pin is
arranged so that its journal is exactly 90° displaced from the main pin. This drives
a Slotted Sector pivoted in the middle, the connecting rod to the valve gear is
driven by a peg running in this sector. By moving the pin from the top to the
bottom of the sector (this is done by the Reversing Gear controlled from the cab)
the valve sequence can be reversed and hence the direction of rotation of the
wheels reversed.