a. It transfers the pressure from trains evenly and widely from the sleepers to the roadbed.
b. It secures the sleepers.
c. It improves drainage.
d. It facilitates the correction of track distortion.
Accordingly, ballast which contains soil cannot be said to fulfill its role to satisfaction, and must be replaced or cleaned by sieving.
As indicated by the results of accelerated ballast sinking tests conducted by the Railway Technical Research Institute (Fig. 4), crushed stone ballast excels in performance over gravel ballast.
Also, thicker ballast yields a better pressure distribution, however, over a specific value, this effect diminishes. Excluding railways with particularly large axle loads, a thickness of 150 - 250 mm is appropriate.
(5) Continuous Welded Rails (CWR)
Rail joints are the weakest point in tracks from the perspective of track deterioration, service life as dictated by rail fatigue, train running, and the generation of sound pollution and vibration. Therefore, it is only natural that we would want to keep the number of joints down to a minimum and eliminate them if possible. Continuous welded rails would have to be employed to eliminate all of the above shortcomings. However, strict requirements apply to curve radius, gradient, bridge interposition, track structures and other factors, and considerable track reinforcement is often needed for implementation. Furthermore, special technologies would have to be acquired for track control, such as rail axle force control and work restrictions during the hot summer months. In locations subject to marginal temperature changes like tunnels, however, implementation can proceed smoothly.
About forty percent of tracks in Japan employ continuous welded rails. The standard length is 1,500 m, and there are expansion joints on both ends. Ultra-long continuous welded rails over 50 km long also exist, and the track circuit consists of glued insulated joints.
As a preliminary stage, welding together currently used rails to as long a length as possible is an effective means of reducing maintenance labor.
Two options are available: rails that have been laid can be welded on site, or rails welded at the factory can be transported, then laid. Several welding methods can be used for both
