When the various works connected with the cuttings and embank ments are finished, and the bridges are ready to receive the ballast, the railway is said to be brought to formation level, and all that remains to be done is to lay down the permanent way. The formation level is usually made about 2 feet below the intended rails level ; the width of its surface is about 30 feet, exclusive of fences and ditches ; and it is made higher in the centre than at the sides, in order to throw off the rain water.
Ballasting and laying the permanent way.---In the earlier periods of the history of railways very warm discussions were carried on with respect to the form of rail, and the mode of attaching it to the sleepers or blocks; but at present there seems to be a very marked tendency on the part of engineers to adopt uniformly the system of double headed rails, keyed to chairs spiked upon cross wooden sleepers, and fish-jointed at the ends. As many useful lessons were learnt in the course of the experiments by which the system now generally adopted was arrived at, and the history of the superstructure of railways is in itself very interesting, a slight notice of the various phases of the discussions in question may he desirable. So much indeed of the commercial success of a railway depends upon the perfection of its permanent way, that it behoves every engineer, at least, to study carefully the phenomena which might have been observed during the respective trials ; and it is even desirable that the directors of railway companies should know something of the previously made experiments in this matter so as to avoid a useless repetition of them hereafter.
When locomotive engines travelling at speeds of even nine miles an hour had been introduced, it was soon found that cast-irou fish-bellied rails were so liable to be displaced and fractured that their use was at once' abandoned; parallel wrought-iron rails were substituted for them universally. On the Liverpool and Manchester, and on nearly all the railways executed by the Stephenson in the early periods of the railway system, the rails were keyed to chairs, fastened to stone blocks bedded in the ballast ; but it was soon found that- the blocks, which were isolated from one another, were exposed to sink unequally, and thus to deform the road; and moreover that the unyielding nature of the point of support (when it had once found a bearing) made the roadway very iujurious to the carriages and rolling plant. Mr. Locke
seems to have been the first to have adopted on a Large scale (as the means of obviating these inconveniences) the use of ,the old system of wooden sleepers, passing beyond the two rails (as shown on jig. 10) so as to carry two chairs; and at the present day this system has been generally adopted wherever the rails are used with only occasional supports. Many essays of cast-iron sleepers have been made, without success; it must be added, for in the present state of iron metallurgy cast-iron goods are too brittle to be employed for articles subjected to the peculiar actions to which sleepers are forcedly exposed ; btit it is still to be hoped that means may be discovered by which this objection may be removed, and that this material, inattackable by insects and comparatively indestructible by atmospheric influences, may be sub stituted for wood. The rapid decay of the wooden sleepers has, indeed, been always the subject of anxiety to railway engineers ; and many patents have been taken out for their preservation. Sleepers have been soaked in solutions of zinc, copper, mercury, &c., with vari able success; but at the present day it seems to be generally admitted that creosote is the material which, when properly applied, contributes the most valuable properties to wood so exposed ; all these pro cesses are however but palliation of a serious evil inherent to the use of wood; and it certainly would be preferable to employ a material exempt from the conditions of decay above enumerated. However this be, the cast-iron chairs used either for blocks or for sleepers are spiked down with iron spikes, and some trifling modifications of the patterns of the chairs are at times advantageously made in order to avoid the splitting of the fibres of the wood. The rails are fastened to the chairs by means of compressed wooden wedges, which naturally expand again when exposed to the effects of moisture ; these wedges are at the present day creasoted on the best lines of railway. Sleepers are usually from 7 to 9 feet long, by 10 in, by 5 in., and are usually placed at distances of 3 feet from centre to centre, on the average; especially on the lines where heavy locomotives are used.