The heavier engine doubtless possesses greater tractive power, but apart from the question of tractive power is the all-important one of steadiness and safety on the rails. A locomotive passing round a curve, even at a moderate velocity, produces disturbances in proportion to the capability of the machine to adapt itself to the altered position, and if both the engine and permanent way are constructed so as to be almost unyielding, then destructive wear and tear and increased risk of derailment must ensue. The adoption of the four-wheel bogie truck to the locomotives on our home and continental lines--although very slow in coming--has contributed greatly to their improvement, enabling the weight to be distributed over a longer, yet more flexible wheel-base, affording greater facility and comparative safety in traversing curves; and rolling, or passing over the rails, 306 with as little injurious effect to them as possible. It is strange to find that the four-wheel bogie truck, originally designed in England in the early days of the railway era, should for so long have met with so little favour on this side of the Atlantic. The Americans, at all times prompt to recognize any appropriate mechanical arrangement, adopted the bogie truck upon its first introduction into the States. They have worked out many improvements in the details, and upon the thousands of locomotives on their vast network of railways, the bogie truck, in one form or another, has been universally adopted from the beginning. On our home and continental lines, the modern express locomotive, with a four-wheel bogie truck in front, is a much longer vehicle than its predecessors, and its total weight is distributed over a greater wheel-base; but the actual weight placed upon the driving-wheels, or on the coupled wheels, is now very much in excess of former practice, and must be taken into consideration when working out the details of girders and cross-girders of under-line bridges. Numbers of girder bridges have had to be taken down and replaced with stronger structures, not for reasons of wear or decay, but simply because they were incapable of carrying with safety the modern heavy rolling loads. Present experience points out the expediency of providing in all new under-line bridges a liberal margin of strength to meet future developments. Figs. 465 to 479 are diagram sketches of a few modern types of locomotives, giving leading dimensions and weights, and may be found useful for reference when working out the necessary strengths of the various portions of bridge-work. Upon comparing some of the principal particulars with those of the earlier class, it will be noted that in many of the modern types the piston area has been doubled, the boiler-pressure doubled, and the weight of the engine doubled also.