Fig. 476 shows an average sample of the ordinary six-wheel-coupled goods engine in use on so many of our home railways. Where the curves are easy and the permanent way strong, the drawback of the long rigid wheel-base may not be so apparent; but for a line abounding in sharp curves, perhaps no more destructive machine could possibly be devised than the ordinary six-wheel-coupled goods engine. Without any flexibility, forced along with great power, and too often driven at unnecessary high speeds, engines of this type have too small a margin of safety when traversing the curved portions of the road. A slight unevenness in the rails, or a sharp flange on the wheel may supply all that is wanting to cause the engine to leave the track, and the probability that such risks are more common than is supposed, is far from satisfactory. The great weight of the engine doubtless tends to keep it on the track, but the rapid wear of the tyres, and of the 312 inside of the rail-heads clearly demonstrate the enormous amount of friction and abrasion that takes place. Fig. 477 represents a type of eight-wheel-coupled engine designed for hauling passenger or goods trains over long lengths of heavy mountain inclines. The engine is a large one in every way, and of great total weight, but the weight is distributed over a long wheel-base and without imposing a greater tonnage per pair of wheels than is done in some of the smaller and less powerful engines. The flanges are turned off the leading pair and third pair of coupled wheels reducing the rigid wheel-base for curves to 9 feet 8 inches. The four-wheel bogie truck in front carries only a moderate weight, being so close to the coupled wheels. Engines of this description require a strong permanent way, as there is a total weight of 60 tons on the four pairs of coupled wheels standing on a wheel-base of 15 feet 6 inches. Figs. 478 and 479 are types of tank-engines in use on some of the narrow-gauge (3 feet) railways. In general design they are somewhat similar to the modern class of engine on main lines of 4 feet 8½ inches gauge, with four-wheel bogie truck in front, and four wheels or six wheels coupled, but with all the parts and weights smaller, to suit the narrow gauge and lighter permanent way.