Another plan which possesses all the advantages of the locomotive described above and is free from the last objection is represented in Plate XVI. This plan was first adopted by Mr. Robert Fairlie in England, but has been introduced into this country and very much improved by Mr. William Mason, of Taunton, Mass. In these locomotives, the driving-wheels and cylinders are attached to a truck frame which turns around a centre-pin like any ordinary truck. The steam and exhaust pipes are connected to the boiler and cylinder with pipes which have flexible joints. By this means the truck can move independently of the boiler, and thus the driving-wheels can adjust themselves to the curvature of the track, just as the wheels of any other truck do, and therefore the driving-wheels can be run ahead just as well as the truck-wheels which carry the tank. This plan possesses the additional advantage that the fire-box can be made as wide and as long as may be desired without interfering with the driving-wheels. The flexible pipes are, however, usually considered an objection; but with the improvements which have been made in their design and construction, the difficulties which were at first encountered have probably been overcome. At any rate if there is no other objection to the use of such locomotives, ingenuity and care should in time overcome that one. Plate XVII represents a locomotive of this plan, with six driving-wheels and a six-wheeled carrying truck under the tank. This latter plan of locomotive is intended for heavy freight traffic. QUESTION 437. _What kind of locomotives is used for passenger trains?_ _Answer._ Eight-wheeled American locomotives are used almost exclusively for passenger service. Usually the driving-wheels of such locomotives are larger in diameter than are used for freight traffic. Their size varies from 5 feet to 5 ft. 9 in. in diameter. The locomotive by the Mason Machine Works represented in Plate VIII has 5¹⁄₂ feet driving-wheels. For very heavy express trains locomotives with 17 × 24 inch cylinders and weighing 34 tons are now used on many through lines. QUESTION 438. _What is meant by suburban and metropolitan railroads, what is the nature of their traffic, and what kinds of locomotives are needed for it?_