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In the construction of all surface railways, the first object considered is the direction of the road, which, in all cases, should be so formed, and with such declivity as may be best calculated not only to suit the nature of the ground through which it passes, but also the trade to be carried on upon it.
If, for example, as frequently happens to be the case, nearly the whole traffic of a country is in one direction, the road should then obviously decline that way, so that the waggons and their contents may descend on this inclined plane as much as possible by their own gravity. But in all cases particular attention ought to be paid to the extent of the trade upon the railway, no that the inclination may be as nearly as possible proportioned thereto, consequently, the draught each way equalized; and in cases where the transit of merchandize to and fro should be nearly equal, it would be most beneficial to have the railway level: but it sometimes may happen that the nature of the ground is such as not to permit that declivity or level best suited to the trade; the line should, in that case, be varied, and, if possible, the inequalities made up so as to bring it as near as possible to the proper standard, if it can be done at any moderate expense; but when the inequalities happen to be such as to resider this impracticable, the only resource to be found is in inclined planes.
For example, where the differences of level between the two extremities of road are such as would resider an equal declivity too steep, then the road must be carried either on a level, or with the due degree of slope as far as practicable, and then lowered by an inclined plane, on which the waggons are gently let down by a brake, and dragged up by an additional power to that which is made use of for drawing them along the road. But in the laying out and formation of all railways, much depends upon the skill and judgment of the engineer, as it is quite impossible to lay down any general plan to suit all cases; for, it must be recollected, every situation presents some peculiar circumstances.
When once the line of railway is fully determined upon, the next step is to form the road, which requires much attention; it must be of sufficient width to contain the opposite rails, and for forming a footpath on one side. There is no prescribed distance between the rails, as, in some cases, preference is given to long narrow waggons, and in others, to those of a broad short shape; consequently, the distance between the rails varies from three to four and a half feet; hence from nine to twelve feet has been usually apportioned for a single road, and from fifteen to twenty for a double one.
The next operation is the placing and firmly bedding the sleepers, which generally consist of solid blocks of stone, weighing from one to two or more hundred weights each. There is no particular shape necessary, provided their bases are broad, and pretty even; it is also particularly necessary that the upper surfaces should present an even and solid basis for the iron plates or rails to rest upon.
The sleepers are generally placed along each side of the road, measuring about three feet distant from each other, from centre to centre, the opposite ones being separated by the width between the opposite rails. In such situations, where the ground under them is of a soft nature, it is usual and proper, in the first instance, to lay on a coat of gravel, small stones, or metallic scoria;; and this is well beaten down in order to form a firm foundation.
Each stone, when laid down, is carefully gauged, both in respect to its distance from the adjoining ones, and the level or declivity of its upper surface, on which the plates or rails are intended to rest. The sleepers being thus correctly placed, the spaces between them are filled up with either gravel, metallic scarier, or some other hard road materials, in order that the whole may consolidate into a hard and firm mass.
The foregoing is a sketch of the process adopted in forming metallic lines of road, whatever may be the form of the rail or wheel-tracks laid down. Of these, there are two principal kinds, namely, tram-plates (already noticed) and edge-rails, both of which are very extensively adopted, though the latter is, at the present time, the most approved by engineers. Nevertheless, it is unquestionable that tram-plates, when correctly formed, and laid down with the same attention to accuracy of adjustment and solidity of bearing as is now practised with the best edge-rails, answer their purpose admirably.
They are commonly employed in Wales, and in the neighbourhood of blast furnaces, on account of the greater facility and cheapness of their construction. They are especially useful in forming new roads, in the working of mines, quarries, in digging canals, in conveying large stones for buildings, and numerous other temporary as well as permanent purposes; chiefly for this reason, that the ordinary wheels of carts and waggons can run upon them, and with a surprisingly increased power of draught, while the carriages are kept steady in their tracks, by the upright flanges, as shown in the annexed section, where B represents the flat bearing surface of the tram-plate, which, as now practised, is fastened by a spike, driven into an oaken plug previously inserted into the stone sleeper C; the horse-path or gravelled road is partly shown at A.
These tram-plates are made of cast-iron, are usually about three feet long, from three to five inches broad, and from half an inch to an inch thick, extending from sleeper to sleeper, and the turn-up flange from two and a half to four inches high. The plate usually bears on the sleepers about three inches at each end, where its thickness is for that purpose increased; between these bearing-points the plate has no support but what it derives from the ground, which, though not very permanent or secure, is infinitely more so than the support thus derived by an edge-rail: indeed, the extensive bearing surface of the plate upon the ground is often found quite sufficient for temporary uses, without any sleepers at all; and in other cases, where a little more stability is required, to spike down the opposite ends of the tram-plates, on each side of the road to a transverse piece of wood, which remains useful for a longer period, without taking up for re-adjustment.
Tram-rails are decidedly of a weak form, considering the quantity of iron in them; and in some works it has been found necessary to strengthen them, by adding a rib on the under side, as shown in the annexed perspective view of a section of half a rail, in which A is the guiding flange; B the bed of the rail on which the wheels run; C is the rib on the under side to strengthen it.
The tram-plates used for repairing the Surrey Tramroad were of this form, and it certainly renders them very stiff.