Joining Track Sections
The sections of rail are welded together using thermite. Conventional welding (using some type of flame) does not work well on large metal pieces such as rails, since the heat is conducted away too quickly. Thermite is better suited to this job. It is a mix of aluminium powder and rust (iron oxide) powder, which reacts to produce iron, aluminum oxide, and a great deal of heat, making it ideal to weld rail.
Before the rail is joined, its length must be adjusted very accurately. This ensures that the thermal stresses in the rail after it is joined into one continuous piece do not exceed certain limits, resulting in lateral kinks (in hot weather) or fractures (in cold weather). The joining operation is performed by an aluminothermic welding machine which is equipped with a rail saw, a weld shear and a grinder. When the thermite welding process is complete, the weld is ground to the profile of the rail, resulting in a seamless join between rail sections. Stress in the rail due to temperature variations is absorbed without longitudinal strain, except near bridges where an expansion joint is sometimes used.
Read more about this topic: TGV Track Construction
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