The HTA wagons form the largest fleet in Porterbrook’s portfolio, being used for bulk coal traffic to supply power stations form mines or ports. The wagon is the modern replacement for the familiar 4-wheel “merry-go-round” wagon (HAA), which was the standard coal wagon on British Rail. Power Stations and pits had their sidings designed for 36-wagon trains of these wagons, hence it is important to fit into this standard train length. The HTA wagons provide the best payload for train length of any existing UK wagons, achieved by reducing the 60 degree hopper angle to 45 degrees. A train of 19 wagons can carry 1,432 tons of payload, compared to 1,206 tons with the old HAA train. The upper body is stainless steel, (which assists in the sliding of coal out of the hopper) and the underframe is mild steel. Bogies are the American NACO three piece “million mile bogie”, a reference to its long overhaul period.