In Japan, composite sleepers made of glass fiber and polyurethane resin have been developed, aiming for a sleeper with the same weight as wooden sleepers and the service life of concrete sleepers. Over a 15-year period, roughly 300,000 composite sleepers have been laid, mainly on bridges. Recent surveys indicate that the goal of a 50-year service life appears feasible.
(3) Rail and Sleeper Fastening
For a considerable period of time, dog spikes and screw spikes were employed as rail and sleepers fastenings. Tie plates are now used to prevent rails from cutting into sleepers and to strengthen resistance to lateral force on curves.
The introduction of the concrete sleeper has given birth to a double elastic fastening with superior absorption capabilities for wheel impact. These fastenings have been acknowledged for features including improved rigidity of track panels and subsequent increased resistance against track buckling, easy management of fastening strength, and effectiveness against electrolytic corrosion for track components on DC electrified sections due to superior electrical insulation. They are now widely used on both concrete and wooden sleepers. Recently, a maintenance-free fastening with no screws was developed, and its use has increased sharply.
(4) Ballast
Ballast serves the following purposes:
a. It transfers the pressure from trains evenly and widely from the sleepers to the roadbed.
b. It secures the positions of sleepers.
c. It improves drainage.
d. It facilitates the correction of track deformation.
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 tracks for particularly large axle loads, a thickness of 150-250 mm is appropriate.
