_Answer._ If a tire on a main driving-wheel or the wheel itself breaks, the driving-box of the broken wheel or tire should be held up by putting a wooden block under the box. An ordinary American engine can then be run on three driving-wheels, but it must be run with the utmost caution. If the engine has more than four driving-wheels there is usually less difficulty in running it, if one of the main wheels is injured, than if there are only four. But it is almost impossible to give directions which will be applicable to all the accidents of this kind that may occur to different kinds of engines. If none of the connecting-rods, crank-pins or crank-pin bosses are injured, it is not necessary to disconnect either side, but if the injury is of such a nature that the coupling-rod must be taken down on one side, the one on the other side must be taken down too. If the main crank-pin and connecting-rod are not disabled, both cylinders may be used even if one of the main wheels or tires is broken. But even if one side of the engine must be entirely disconnected, the engine may still be run with one cylinder and by driving one wheel. If a main axle breaks the engine can usually be run, but great caution must be exercised. In such cases, however, if assistance or a telegraph office is near where the accident occurs, it is usually best to send for assistance at once, rather than take the risks which attend the attempt to run an engine so seriously injured. QUESTION 524. _What must be done when a trailing or leading driving-wheel, tire or axle breaks?_ _Answer._ Very much the same course must be pursued as was described in the previous answer, although it is generally less difficult to run with a trailing or leading axle broken than it is when the main axle has met with such an accident. QUESTION 525. _What must be done if an engine-truck wheel or axle breaks?_ _Answer._ It is usually best to chain up the end of the truck-frame over the broken axle or wheel to the engine frame and place a cross-tie across the other end of the truck-frame, between it and the engine frame, so that the weight of the engine may rest on the cross-tie. If a part of the flange or a piece of the wheel is broken out, the wheels should be turned around so that the unbroken part will rest on the rail, and they should then be chained or otherwise fastened so that they cannot revolve, and thus be made to slide on the rails and carry the weight of the engine in that way. The same plan is employed if a tender wheel breaks, but one end of a tender-truck frame must be chained up. It is usually necessary to place a cross-tie across the top of the tender, and fasten the chains to it. QUESTION 526. _What must be done in case a driving-spring, spring-hanger or equalizing-lever breaks?_