Returning to the leading bogie some detail can be explained. The centre wheel is present on one side only. Careful observation will reveal that, in this view, it was not in contact with the rail and that, unlike the other wheels it has neither cone shaped tread nor flange. If you look directly above, along the line of the hand-rail, a hand wheel can be just seen through the window. This horizontal hand wheel is used to lower the measuring wheel into contact with the rail at the start of the test.