306 RAILWAY CONSTRUCTION. 



passing over the rails, 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 pro- 

 viding 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 com- 

 paring 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. 



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. 



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 



