Standard ballast sections have been adopted in recent years which provide a greatly increased depth of ma terial beneath the ties. For the heaviest service this depth is now no less than 24 in., which is based on the principle that a roadbed material subject to deformation by the application of a live load should have a depth sufficient to produce approximately uniform pressure on the roadbed, which is thus the maximum spacing be tween ties. The heavy expense for ballast of this depth is avoided by the use of a sub-ballast of a material superior to that of the roadbed, and often equally efficient with the ballast in its two functions of drainage and distribution of the lead. Crushed stone and gravel are the materials most generally employed for ballast.
The former is regarded as superior to all other materials for this purpose, since its angular fragments bind together so as to dis tribute the load over the largest area, and it affords the best drainage for the longest period without cleaning or renewing. Washed gravel is nearly as effective for ballast as crushed stone.
and is much cheaper where it is available. Burnt clay, slag and cinders are materials in common use for ballasting the less im portant tracks. The cleaning of ballast, which was formerly done by hand labour with ballast forks, is frequently done with mechanical equipment.
Ties.—While the general design of the track has not changed materially since the early days of railroads, the size and spacing of the ties, the weight and length of the rails, the strength and rigidity of the rail fastenings, and the designs of standard and special trackwork, have undergone a progressive de velopment. Among the earlier expedients for strengthening the track structure was a closer spacing of the ties and, more re cently, the specifying of wider ties than those standard in pre vious years. Whereas 7 in. ties were considered adequate 25 years ago (although doubtless many wider ones were used) present practice favours an 8 or 9 in. width for heavy-traffic main tracks. Similarly, a spacing of 16 ties to the 33 ft. panel was then cus tomary, while 20 ties to 33 ft. is now nearly universal. These two features have increased by about 5o% the extent of the rail directly supported on the ties. Probably no fact regarding ties is more striking than the attention which is being given to their conservation by an increasing use of tie plates to protect them against mechanical wear and by a more extensive use of chemical preservatives, approximately three-fourths of all ties used by steam railroads now being so treated. These protective measures
have also conserved timber (see TIMBER PRESERVATION) by enabling the faster-growing woods to be used for ties.
Development in this field has been outstanding in recent years. The weight of rail, which about 1900 was frequently no heavier than 6o lb., has been more than doubled. Recent studies have shown the trend on the part of the railways toward heavier sections for their main-line tracks. The 136 lb. rail is the heaviest section used in any main-line American track. Important changes have also taken place in the design of the rail section, as well as in the composition of the steel from which the rails are rolled. In American Railway Association sec tions, developed in 1905, the height was generally one-tenth greater than the base, with the sides sloped on an angle of 1 in 16, which slope is retained in the designs recently adopted by the American Railway Association and known as the R. E. sections. By reason of the uniformity now possible in its manufacture, rail steel has tended toward a generally higher carbon content than formerly, the specifications adopted by the American Railway Association permitting as high as 0.89%. (See IRON AND STEEL for a discussion of manufacture.) The standard length of rails, which formerly was 3o ft., is now 39 feet. While rails as long as 6o ft. were laid 25 years ago, the practicability of the longer rails has only been possible through the development of an im proved joint structure, together with devices for restraining the creeping of the rails, along with a higher standard of roadway design and better maintenance of the roadbed structure. Joint fastenings, which formerly were of light angle bars, with fish plates not uncommon in certain tracks, have had their section greatly increased, while a number of designs have been developed looking to a closer approach of the joint to the strength of the unbroken rail. The general use of heat-treated or high-tensile bolts, and of spring washers of the higher tensions, has also con tributed greatly to the rigidity of the joint structure. Auxiliary track fastenings, such as tie plates and anti-creepers, are being made in heavier designs and used more plentifully in all classes of railway tracks.