_Answer._ The ordinary American eight-wheeled locomotive is used more than any other kind; but a number of locomotives like that represented by fig. 222 have been built and are used for this traffic. These have one pair of driving-wheels in front of the main pair and a Bissell truck in front of the cylinder. With this arrangement the driving-wheels bear a larger proportion of weight than they do if arranged on the ordinary American plan with a four-wheeled truck. Another plan is that shown in Plate XVIII. Such engines, as will be seen, have a Bissell truck at each end, and therefore they run equally well either way. In some cases the tanks of such engines are carried on the top and sides of the boiler. When they are obliged to run only a short distance, and a small supply of water is needed, this arrangement answers very well; but it is impossible to carry a large supply of water in this way without overloading the wheels of the locomotive, and at the same time increasing the evils of a varying load on the driving-wheels. [Illustration: Fig. 222. LIGHT PASSENGER AND FREIGHT LOCOMOTIVE, BY THE GRANT LOCOMOTIVE WORKS, PATERSON, N. J.] Locomotives like that shown in Plate XIV are also used for suburban traffic. As shown in the engraving they have a four-wheeled truck at one end and one with two wheels at the other, so that it is thought that they can be run safely either way. The four-wheeled truck carries the weight of the water and fuel. The plan of engine represented by Plate XV is very well adapted for this kind of traffic. Excepting on curves with a very short radius it could be run in either direction at any required speed, without encountering any other difficulty excepting the prejudices of those who run it. As double-truck locomotives similar to that shown in Plate XVI can adjust themselves to any curve, this objection could not be urged against their use. QUESTION 440. _What kinds of locomotives are used on metropolitan railroads?_