Stanesby, “Railways”, part 2

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Designing a Line of Railway.—It is not intended here to enter into a disquisition on the important economical questions which should be considered in marking out the main lines of communication in a country. It is the opinion of many persons that a system of railroads should be laid out by the government of a country, whether they are actually formed by the state or by private individuals. Arguments in support of such a view have been drawn from the want of unity of plan which is evident in the railways of England, most of which have been designed in short lines from one important town to another, without due regard to combination of plan. The commissioners appointed to report on a system of railways for Ireland have considered this subject very ably, and endeavoured, in their proposed lines, to avoid the errors consequent on the limited views of private speculators. Most of the continental railways have been laid out more under government control than those of England, but there are not at present sufficient data from experience to allow of a fair comparison between the working of the two systems. In considering this point, it should not be forgotten that, however desirable a comprehensive plan may be in a country yet to be supplied with railways, experience in cases most analogous leaves but little reason for supposing that the railway system would have made the sudden advances that it has, unless under the stimulating, though by no means unexceptionable, agency of private speculation and commercial enterprise.

When the termini and general course of a line of railway are determined on, it remains carefully to examine the country to be passed over,—its elevations and depressions—its rivers, canals, roads, and all other streams of water or means of communication that have to be crossed, or in any way interfered with—and its geological structure; any of which may occasionally render a deviation from the direct course advisable.

As a general rule, a perfectly straight and level line is to be preferred when the termini are of equal elevation, or a uniform slope when one is higher than the other. An attempt has indeed been made to prove that a railway formed in a series of undulations would be preferable to one perfectly level, because the power of gravity might be used to aid in the descents, and that of acquired momentum in the ascents, thereby reducing the amount of artificial power required for moving carriages upon the road; but the general opinion of engineers is not favourable to this theory. There are, however, some circumstances under which advantage may be taken of the powers of gravity and momentum, without the serious inconveniences which would attend the use of an undulating railway. But as it rarely happens that a perfect level or a uniform slope can be attained for any great distance without such a deviation from the natural surface of the ground as would be very inconvenient and expensive, the engineer so adjusts his inclinations or gradients as to make the nearest practicable approach to a level; avoiding if possible any loss of power from undulations of surface, by making all the inclinations on one side of the summit, or highest point to he passed over, rise towards it, and all on the opposite side descend from it. In order to the due adjustment of the gradients, a section or profile of the line of country is prepared, in which the elevations and depressions are drawn to a much larger scale than the horizontal distances. Fig. 12 is a section of an imaginary line, resembling those prepared for parliamentary inspection. The horizontal line at the bottom is given as a basis for measuring the elevations from, and is made to have reference to some fixed point near one of the termini. This section may be supposed to represent the line of a railway between a seaport town at A, and an inland town at F: the undulating line representing the natural surface of the ground; the straight lines from point to point, the intended surface of the railroad; and the vertical lines marking the changes of inclination. Owing to the intervening high ground, a uniform slope from A to F is impracticable, but a line with very moderate inclinations is obtained by tunnelling through the ridge at i, excavating the minor elevations, and filling up the hollows. If a road were made on the natural surface of the ground, a carriage passing along it would, after mounting to the elevation g, have to descend to h, and immediately remount to the top of i. But in a road constructed on the level of the proposed railway, not only would part of the elevation of i be avoided by the tunnel, but that which remains would have to be ascended only once, as every part between A and d, the summit of the road, rises towards it, though in different degrees; and in like manner the whole distance between d and e inclines downward, while the remaining part, from e to F, is perfectly level.

Owing to the short interval which has elapsed since the commencement of railway operations on a large scale, many theoretical points respecting them yet remain unsettled. Even the amount of retarding effect caused by passing over a given {70} elevation is variously estimated by different engineers. On an ordinary road the resistance arising from friction and irregularity of surface is so great that the effect of gravity is scarcely perceptible on a moderate inclination; but on a railway the friction and road-resistance are reduced to so small an amount, that gravity, which remains the same, becomes a material part of the total resistance, even where the inclination of the road is very slight. It is a theory of many engineers, that an elevation of twenty feet requires an exertion of power equal to that on a mile of level railway; so that the same power which would move a given load over one mile of railway rising 1 in 264, or twenty feet in the whole, would move the same load over two miles of level road. The practical importance of this question is very great, because a correct understanding of it is essential to show how far it may be advisable to deviate from a direct course in order to avoid a given elevation. Supposing, for instance, that a railway is required between two points twenty miles apart, and that a straight course may be obtained by passing over an elevation of 100 feet, it may be preferable to increase the length to twenty-four miles, if by so doing a level can be obtained; because the elevation of 100 feet will require as great an expenditure of power as five miles of horizontal railway.

It is often necessary to conduct a railway over a considerable elevation, but engineers differ as to the best arrangement of the unavoidable inclinations. Some prefer distributing the rise and fall as equally as possible throughout the whole line, while others consider it best to concentrate them in a few steep planes, in ascending which additional power is used, and to make the rest of the line comparatively level. The Liverpool and Manchester Railway may be cited as an instance of the latter mode, the main line having no gradient exceeding 1 in 849, with the exception of two inclined planes of about a mile and a half each, inclining 1 in 89 and 1 in 96, near Rainhill, at which it is usual to assist the trains by an additional locomotive engine. The Great Western Railway also, in a length of 117½ miles, has no steeper gradient than six feet six inches per mile, or about 1 in 812; excepting two inclined planes of 1 in 100. In the London and Birmingham Railway, which affords an example of the former system, the ordinary gradient is 1 in 330, or sixteen feet per mile, which is nowhere exceeded except on the extension from Camden Town to Euston Square. The characteristic or ordinary gradient on the South-Western, Brighton, South-Eastern, and many other lines, is 1 in 264, or twenty feet per mile.

A certain degree of similarity in the gradients is essential to the economical working of a railway by inanimate power. If any inclination occur so steep that the ordinary power cannot ascend it by a reduction of speed, it must either be surmounted by the aid of auxiliary power, or the engine must run over other parts of the road with less than a maximum load, and consequently at unnecessary expense. So long as this inconvenience is avoided, it is the opinion of some scientific men that the degree of inclination is of little consequence on a railway with an equal traffic in both directions, because the assistance of gravity in the descent, being set against the additional resistance in ascending, brings the total amount of power required in traversing the line in both directions to nearly the same as would be needed if the road were a perfect level.

Some highly interesting experiments have been recently made on this and other points of railway economy, under the superintendence of Dr. Lardner, of which the following seems to indicate that this compensating effect takes place on inclinations of much greater steepness than has been generally supposed. Great caution is necessary in forming calculations on such a subject from single experiments, however carefully conducted, but the results are certainly such as to justify serious inquiry. In July, 1839, the Hecla engine, with twelve carriages, making a gross weight, including the engine, of eighty tons, was run from Liverpool to Birmingham and back in the same day, by which means the same train, under as nearly as possible the same circumstances, had to ascend and descend every plane on the line, a length of about ninety-five miles. The time of passing each quarter-mile was carefully observed, so as to obtain the speed on every portion of the road. The following table, extracted from the seventh edition of ‘Lardner on the Steam-Engine,’ gives the result of observations on gradients varying from level to 1 in 177, or nearly thirty feet per mile:—

From this table it appears that, although the plane of 1 in 177 diminished the speed from near thirty-one miles per hour, the velocity on a level, to little more than twenty-two miles, in the ascent, the deficiency was fully compensated by the increased rapidity in the descent. The result fairly indicates a most remarkable and valuable fact—namely, that a line of railway with gradients of from twenty to thirty feet per mile may be worked in both directions by the same expenditure of power as a dead level; and this fact, if substantiated by more extended experiment, proves that many millions may be saved in the execution of future railways by being content with steeper inclinations than have hitherto been considered advisable. The whole of the compensating effect here produced is not to be attributed to the agency of gravity and momentum—a part, and perhaps a very considerable part of it, being due to the diminished resistance of the air to the passing of the train on ascents, owing to its reduced velocity. The nature and extent of atmospheric resistance to railway trains are things on which so little is known, and opinions are so conflicting, that the extent of its influence in the experiment alluded to cannot be stated with certainty, but it is probably considerable, as the result is very different from that which might by calculation have been expected from the mere effect of gravity and friction. The resistance of the air being almost imperceptible in the case of common roads, owing to the great friction and moderate velocity, has frequently been considered too trifling to become an element in calculations on railway transit, and hence arises much of the error that has hitherto prevailed respecting inclined planes.

Curves on a main line of railway being very objectionable, a judicious engineer so adjusts his line as to avoid them when possible, and to make those which are inevitable of as large a radius as circumstances will admit. Curves of less than a mile radius are considered unadvisable for places where great velocity is required, although many of only half-a-mile radius are in use. At stations and depots, where the trains always move slowly, the radii may be much shorter without inconvenience.

A railway should not be allowed to cross any much-frequented road on the same level. When the Liverpool and Manchester line was projected, as the rate of travelling was not expected to exceed ten miles per hour, no danger was anticipated from such intersections, which are called surface-crossings; and accordingly several were allowed: but their inconvenience and danger have caused some of them to be altered. In recent railway acts it is enacted that no turnpike-road or highway shall be crossed on the same level; a rule to which exceptions are very rarely allowed; and if they are, gates must be erected to enclose the railway, and attendants stationed to open them for the passage of vehicles across it. These gates should be so hung as to completely close the railway when the road is open, and vice versâ. In a few instances two railways have been allowed to intersect each other on the same level, but this highly dangerous arrangement is now very rarely permitted. Where a single road is crossed, {71} it may not be necessary to regard it much in selecting the level for the railway, as such road may be made to slope gradually to the requisite level for passing under or over it; but in approaching towns, where many communications are interfered with, it is essential that the railway level should be made higher or lower than the ordinary surface, in order to avoid them. At Liverpool this is effected by tunnels under the town; at the London end of the Birmingham Railway by an open cutting; and at Manchester, Birmingham, and many other places, by an embankment or viaduct. The Greenwich Railway, extending over a metropolitan district the whole of its length, is entirely on a viaduct, and that from London to Blackwall, a similar line, is principally so.

Railways frequently intersect the course of rivers and canals, and numerous bridges are necessary. Where the course of the streams thus crossed is sinuous, expense may sometimes be reduced by making a new channel for the river, such a cut often being the means of avoiding the erection of two bridges, as in the instance of the Manchester and Leeds Railway in the valley of the Calder.