Our illustration (Plate XIX.) shows an express goods engine with six wheels coupled, cylinders 18 inches by 24 inches, and wheel diameter 5 feet.
Plate XX. shows a special tank engine for goods trains, having six wheels coupled, 4 feet 3 inches diameter, and cylinders 17 inches by 24 inches.
Plate XXI. is an engine used for drawing coal trains, having 6 wheels coupled, 4 feet 3 inches diameter, and cylinders 17 inches by 24 inches.
It was formerly the practice to work the express passenger trains with single engines of the "Lady of the Lake" class (see Appendix), although, later, it was found better to work the heavier trains with four-wheeled coupled engines; but these, having proved to be too small for the increasing loads and the higher rate of speed demanded, are now being superseded by compound engines of a new construction, invented by Mr. Webb, of which the "Marchioness of Stafford" (Plate XVIII.) is an example, and which may be thus described: The engines differ from the compound engines in use on other lines chiefly in the number and disposition of the cylinders, there being two high-pressure cylinders fixed outside the frames, between the leading and middle wheels (the connecting rods working on to crank pins, set at right angles to each other, in the trailing wheels), and one low-pressure cylinder carried between the main frames at the front end of the engine, the connecting rod working on to a single throw crank in the middle pair of wheels. For the benefit of non-professional readers, it may be explained that the underlying principle of a compound engine is that the steam, instead of being allowed to escape after having once done duty, is compelled, by an arrangement of
