The new rolling stock was in three-car units of which there were 19, with steel bodies and open saloon accommodation. They had air-operated sliding doors, and automatic couplers. There was one motor coach in each three car unit, and it was equipped with four 135 hp motors. Additional power was considered to be required for the steep gradients in the Mersey Tunnel. Electrical equipment was supplied by British Thomson-Houston, and the vehicles were manufactured by Metropolitan Carriage and Wagon Company and the Birmingham Railway Carriage and Wagon Company. The units later became British Rail Class 503.