_Answer._ It is impossible to give any directions for replacing locomotives on the track which will meet the great variety of circumstances which occur in practice. If the engine has not run far from the rails, it can usually be run on again by placing blocks of wood under the wheels and thus running them up to their proper position, but if the engine falls on its side or runs down an embankment, it is usually necessary to send for the appliances which are now provided on nearly all roads for removing wrecks and replacing engines on the track. These appliances are generally stored in what is called a wrecking or tool car, which is placed at a convenient point on the road, from which it can be sent to any place where its services are likely to be needed. Such cars are generally provided with ropes, jack-screws, chains, crowbars, levers, etc., to be used in such cases, and generally a special set of men is sent with the wrecking car to direct and assist in replacing engines and cars on the track. It would lead us too far to describe all the methods of doing this employed under various circumstances; and as such work seldom forms part of the duties of a locomotive runner, a complete description would be out of place here. QUESTION 511. _After an accident which disables the engine, what is the first thing to do?_ _Answer._ The first thing to do is always to “_protect the train_;” that is, to send out signal men in each direction to stop approaching trains; otherwise they might run into the wrecked train, and thus cause a double accident. QUESTION 512. _What is the chief cause of boiler explosions?_ _Answer._ The cause of all boiler explosions, as happily expressed by a prominent American engineer,[115] is THAT THE PRESSURE INSIDE THE BOILER IS GREATER THAN THE STRENGTH OF THE MATERIAL OUTSIDE TO RESIST THAT PRESSURE. This may occur in two ways: first, and most frequently with locomotives, from insufficient strength of the boiler to bear the ordinary working pressure; and second, from the gradual increase of heat and pressure until the latter is greater than the boiler was calculated to bear. [115] See Fifth Annual Report of the American Master Mechanics’ Association, page 196. Insufficient strength may be due: 1, to defects of the original design, owing to the ignorance of the strains to which the material of the boiler will be exposed, and its power of resistance; 2, to defective workmanship and material, which can usually be discovered by careful inspection; 3, to the reduction of the original strength of the boiler by ordinary wear and tear or neglect, which can also usually be discovered by careful inspection. The first two causes have been fully discussed in the part relating to boiler construction, and the last under the head of inspection of locomotives.