_Answer._ The greater part of the freight service of this country is performed by locomotives like that selected for the illustrations of these articles, and represented in Plates I, II and III. Such locomotives have been called “American” locomotives because they first originated in this country and are now more generally used here than anywhere else. Side elevations of locomotives of this kind, built by the Baldwin Locomotive Works, the Grant Locomotive Works, the Danforth Locomotive and Machine Company, the Mason Machine Works and the Hinkley Locomotive Works are represented in Plates V, VI, VII, VIII and IX. Such locomotives have been described in the preceding pages. QUESTION 432. _What are the dimensions of such engines?_ _Answer._ The principal dimensions of the engines illustrated are given in the table in the appendix, but locomotives of this plan are built of much smaller and also of larger sizes than those represented by the engravings. In some cases such locomotives do not weigh more than 35 or 36,000 lbs., with cylinders from 8 to 12 inches in diameter. In other cases they weigh as much as 66,000 lbs., with cylinders 17 or 18 inches in diameter. The wheels vary from 4 to 6 feet in diameter, but the most common sizes are 4¹⁄₂ and 5 feet. QUESTION 433. _When it is desirable to pull heavier loads than is possible with the adhesive weight that can be placed on four driving-wheels, what is done?_ _Answer._ One or more pairs of driving-wheels are added, as in the ten-wheeled locomotive represented in Plate X, the “Mogul” engine, Plate XI, and the “Consolidation” engine, Plate XII. The ten-wheeled locomotive, it will be seen, is similar in construction to an ordinary American locomotive, excepting that it has another pair of driving-wheels in front of the main driving-wheels. It will be seen, however, that it is necessary to keep these close to the latter, because if they are brought further forward they will be too near the back truck-wheels. For this reason a truck consisting of a single pair of wheels, _A_, _A_, is placed in front of the cylinders, as represented in Plates XI and XII, is now much used. The front driving-wheels are then placed further forward, and thus bear a larger proportion of weight than they do in locomotives like that shown in Plate X. QUESTION 434. _How are trucks with a single pair of wheels constructed?_