The cylinders were placed above the center driving axle. This system used a beam whose fulcrum was the driving axle. The third driving axle was left unpowered. The biggest problem was the one-piece driving axle. Its defining feature is that the firebox is placed "behind" the rearmost driving axle. The boiler was placed entirely "underneath" the driving axle. The barrel-chested E-6 was the first locomotive to achieve over 1000 HP per driving axle. Because the single driving axle was behind the firebox, Crampton locomotives usually had outside cylinders. The carrying axle and the first driving axle were housed in a Krauss-Helmholtz bogie. The 4-2-2 locomotive had one single driving axle.