Basic Components of Track
Today most railways have rolling stock with hard steel flanged wheels running on two rails set at or about 1432mm standard gauge, supported in some way to spread loads to the ground below.
?Sub-grade' is the term used for the natural s soil, after trimming off organic topsoil and made ground, upon which the track bed is constructed.
The ?Trackbed' comprises-ballastlayerstheandis ballast and any there to support the track, to drain water from the bottom of the sleepers
and to distribute the imposed track load to such a degree that the sub-grade can resist the imposed bearing pressure adequately.
Early railway engineers did not at first realise the important engineering function carried out by the ballast, as outlined above. Because of this all manner of material was used beneath the sleepers which today would be considered completely unsuitable. This included materials which would be cheaply and easily available locally such as ashes, chalk and clay. Experience soon showed that good quality ballast, made of well graded gravel, crushed gravel, limestone or igneous rock was necessary if adequate foundation and good drainage is to be achieved for a reasonable period. Additionally it was found that, even with good quality crushed material, the presence of a high proportion result in silting up and softening of adjacent and supporting materials, thus causing track settlement and drainage long term problems.
Today the required depth of good quality ballast beneath sleepers varies depending upon the maximum speed of trains, the maximum axle loads carried and the gross annual tonnage expected. In general the absolute minimum depth of ballast needed beneath sleepers for even a lightly loaded railway should never be less than 150mm and heavily loaded main lines can require as much as 280 mm. The currently recommended minimum thicknesses of ballast beneath sleepers for lines in the UK are as shown in To ensure both lateral and longitudinal stability of the track, particularly when using continuously welded rail, it is essential that ballast is taken up to the level of the top of the sleepers between the sleepers and given a good ?shoulder' at the this condition, regular inspection and periodic tamping is necessary.
Materials for Track Ballast
Good quality track ballast is made from crushed natural rock with particles not larger than 50mm nor generally smaller than 28mm. Angular stones are preferable to naturally rounded stones, to achieve the best interlock properties and resistance to longitudinal and lateral movement under dynamic loading. If ballast particles are larger than the maximum size stated there may only be two or three stones between the underside of the sleeper and the sub-grade which will be insufficient to properly distribute the load. Too many small stones below 28mm will however clog the ballast and reduce, in the longer term, its drainage properties. Samples of track ballast must be checked for grading by sieve analysis. Not more than 3% by weight should be retained on the 50mm square mesh sieve and not more than 2% should pass through the 28mm sieve.
Ballast particles can suffer degradation due to the action of traffic and maintenance operations in broadly two ways. Either edges can become rounded and lose their interlocking effect or particles can break or crush under repeated loading. Some of the softer stones suffer badly from attrition in the presence of water. This deterioration, particularly at rail joints, can be associated with ?wet spots' of line, level and riding comfort.
Certain tests can be introduced to check the wet attrition qualities of ballast. Generally speaking limestones tend to have poor wet attrition qualities, and crushed granite being one of the best, although expensive.