The engines shown in Figs. 465 and 467 have great weights placed on the single driving-wheels, and should only be used where there is a very strong permanent way. With the four-wheel coupled engines, the weight for adhesion can be distributed between the driving and trailing wheels. [Illustration: Fig. 468, 469, 470] [Illustration: Fig. 471, 472, 473] Fig. 473 represents a very excellent type of American engine which has been extensively adopted in the United States for many years. The six coupled wheels distribute the weight over a fairly long wheel-base, 309 retaining their united weight for adhesion. The four-wheel bogie truck in front forms a valuable path-finder to the engine, both for passing round curves or on straight line. This class of engine is very serviceable for various kinds of traffic, and is particularly suitable for lines where the rails and fastenings are comparatively light. In the example shown, the flanges are turned off the centre pair of coupled wheels; but for lines where the curves are of small radius, the flanges may be turned off the leading pair of coupled wheels, instead of the centre pair, to reduce the length of rigid wheel-base. This type of engine has latterly been introduced on various European and foreign railways, and recently on the Highland Railway of Scotland, as shown in Fig. 470. The writer has had engines of this class under his charge abroad, and found them to be most useful for heavy passenger and goods-train service. They run very steadily, are easy on the permanent way, and light in repairs. As they become better known they will be more appreciated, and will doubtless before long supersede in many cases the rigid six-wheel-coupled goods engine. The principal objection of any importance that can be raised against them is that on many lines the present engine turn-tables are too small for such long engines; but it would be far more economical in the long run to enlarge a few turn-tables than to continue the adoption of rigid engines which from their form and arrangement tend to unnecessary wear to themselves and the permanent way. [Illustration: Fig. 474, 475, 476] [Illustration: Fig. 477, 478, 479]