 The entire ensemble was placed on a common frame and thus often referred to as an 0-6-6-0 tank engine, even though the actual notation is 0-6-0+0-6-0 since both engine units pivoted as on a Garratt, Double Fairlie or Meyer locomotive.The Leader prototype was constructed at Brighton railway works, work beginning in 1946.An initial order for five locomotives was placed straight from the drawing board in 1946 and a further 31 were ordered in 1947, although, with nationalisation looming, this was merely a gesture.