It so chanced that it was the Smith vacuum brake which failed to work at Communipaw, and the Eames vacuum which failed to work at Franklin street. This, however, was wholly immaterial. It might just as well have been the original Westinghouse. The difficulty lay, not in the maker's name, but in the imperfect action of the brake; and such significant intimations are not to be disregarded. The chances are naturally large that the failure of the continuous brake to act will not at once occur under just those circumstances which will entail a serious disaster and heavy loss of life; that, however, if such intimations as these are disregarded, it will sooner or later so occur does not admit of doubt.
But the possibility that upon some given occasion it might fail to work was not the only defect in the original Westinghouse; it might well be in perfect order and in full action even, and then suddenly, as the result of derailment or separation of parts, the apparatus might be broken, and at once the shoes would drop from the wheels, and the vehicles of the disabled train would either press forward, or, on an incline, stop and run backwards until their unchecked momentum was exhausted. This appears to have been the case at Wollaston, and contributed some of its most disastrous features to that accident.
To obviate these defects Westinghouse in 1872 invented what he termed a triple valve attachment, by means of which, if the thing can be so expressed, his brake was made to always stand at danger. That is, in case of any derangement of its parts, it was automatically applied and the train stopped. The action of the brake was thus made to give notice of anything wrong anywhere in the train. A noticeable case of this occurred on the Midland railway in England, when on the November 22, 1876, as the Scotch express was approaching the Heeley station, at a speed of some sixty miles an hour, the hind-guard felt the automatic brake suddenly self-applied. The forward truck of a Pullman car in the middle of the train had left the rails; the front part of the train broke the couplings and went on, while the rear carriages, acted upon by the automatic brakes, came to a stand immediately behind the Pullman, which finally rested on its side across the opposite track. There was no loss of life. On the other hand, as the Scotch express on the North Eastern road was approaching Morpeth, on March 25, 1877, at a speed of some twenty-five miles an hour, the locomotive for some reason left the track. The train was not equipped with an automatic brake, and the carriages in it accordingly pressed forward upon each other until three of them were so utterly destroyed as to be indistinguishable. Five passengers lost their lives; the remains of one of whom, together with the wheels of a carriage, were afterwards taken out from the tank of the tender, into which they had been driven by the force of the shock.
The theoretical objection to the automatic brake is obvious. In case of any derangement of its machinery it applies itself, and, should these derangements be of frequent occurrence, the consequent stoppage of trains would prove a great annoyance, if not a source of serious danger. This objection is not sustained by practical experience. The triple valve, so called, is the only complicated portion of the automatic brake, and this valve is well protected and not liable to get out of order.[24] Should it become deranged it will stop the working of the brake on that car alone to which it belongs; and it will become deranged so as to set the brake only from causes which would render the non-automatic brake inoperative. When anything of this sort occurs, it stops the train until the defect is remedied. The returns made to the English Board of Trade enable us to know just how frequently in actual and regular service these stoppages occur, and what they amount to. Take, for instance, the North Eastern and the Caledonian railways. Both use the automatic brake. During the last six months of 1878 the first ran 138,000 train miles with it, in the course of which there were eight delays or stoppages of some three to five minutes each occasioned by the action of the triple-valve; being in round numbers one occasion of delay in 17,000 miles of train movement. On the Caledonian railway, during the same period, four brake failures, due to the action of the triple-valve, were reported in runs aggregating over 62,000 miles, being about one failure to 15,000 miles. These failures moreover occasioned delays of only a few minutes each, and, where the cause of the difficulty was not so immediately apparent that it could at once be remedied, the brake-tubes of the vehicle on which the difficulty occurred were disconnected, and the trains went on.[25] One of these stoppages, however, resulted in a serious accident. As a train on the Caledonian road was approaching the Wemyss Bay junction on December 14th, in a dense fog, the engine driver, seeing the signals at danger, undertook to apply his brake slightly, when it went full on, stopping the train between the distant and home signals, as they are called in the English block system. After the danger signal was lowered, but before the brake could be released, the signal-man allowed a following train to enter upon the same block section, and a collision followed in which some thirteen passengers were slightly injured. This accident, however, as the inspecting officer of the Board of Trade very properly found, was due not at all to the automatic brake, but to "carelessness on the part of the signal-man, who disregarded the rules for the working of the block telegraph instruments," and to the driver of the colliding train, who "disobeyed the company's running regulations." It gives an American, however, a realizing sense of one of the difficulties under which those crowded British lines are operated, to read that in this case the fog was "so thick that the tail-lamp was not visible from an approaching train for more than a few yards."
After the application of the triple valve had made it automatic, there remained but one further improvement necessary to render the Westinghouse a well-nigh perfect brake. A superabundance of self-acting power had been secured, but no provision was yet made for graduating the use of that power so that it should be applied in the exact degree, neither more nor less, which would soonest stop the train. This for two reasons is mechanically a matter of no little importance. As is well known a too severe application of brakes, no matter of what kind they are, causes the wheels to stand still and slide upon the rails. This is not only very injurious to rolling stock, the wheels of which are flattened at the points which slide, but, as has long been practically well-known to those whose business it is to run locomotives, when once the wheels begin to slide the retarding power of the brakes is seriously diminished. In order, therefore, to secure the maximum of retarding power, the pressure of the brake-blocks on the revolving wheels should be very great when first applied, and just sufficient not to slide them; and should then be diminished, pari passu with the momentum of the train, until it wholly stops. Familiar as all this has long been to engine-drivers and practical railroad mechanics, yet it has not been conceded in the results of many scientific inquiries. In the report of one of the Royal Commissions on Accidents, for instance, it was asserted that the momentum of a train was retarded more by the action of sliding than of slowly revolving wheels; and again, as recently as in May, 1877, in a scientific discussion in London at one of the meetings of the Society of Arts, a gentleman, with the letters C. E. appended to his name, ventured the surprising assertion that "no brake could do more than skid the wheels of a train, and all continuous brakes professed to do this, and he believed did so about equally well." Now, what it is here asserted no brake can do is exactly what the perfect brake will be made to do,—and what Westinghouse's latest improvement, it is claimed, enables his brake to do. It much more than "skids the wheels," by measuring out exactly that degree of power necessary to hold the wheels just short of the skidding point, and in this way always exerts the maximum retarding force. This is brought about by means of a contrivance which allows the air to leak out of the brake cylinders so as to exactly proportion the pressure of the blocks on the wheels to the speed with which the latter are revolving. In other, and more scientific, language the force with which the brake-blocks are pressed upon the wheels is made to adjust itself automatically as the "coefficient of dynamic friction augments with the reduction of train speed." It hardly needs to be said that in this way the power of the brake is enormously increased.
In America the superiority of the Westinghouse over any other description of train-brake has long been established through that large preponderance of use which in such matters constitutes the final and irreversible verdict.[26] In Europe, however, and especially in Great Britain, ever since the Shipton-on-Cherwell accident in 1874, the battle of the brakes, as it may not inappropriately be called, has waxed hotter and hotter; and not only has this battle been extremely interesting in a scientific way, but it has been highly characteristic, and at times enlivened by touches of human nature which were exceedingly amusing.