Rail joints are the weakest point in tracks from the view point of track deterioration, service life as dictated by rail fatigue, smooth train running, and the generation of noise 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 have been introduced to eliminate all of the above shortcomings. However, in introducing welded rails, strict requirements must be applied to curve radius, gradient, bridge interposition, track structures and other factors, and considerable track reinforcement is often needed for its implementation. Furthermore, special technologies would have to be acquired for track control, such as rail axial 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 1,500m, and there are expansion joints on both ends. Ultra-long continuous welded rails over 50 km long also exist, and non-insulation track circuit is used.
Two options for laying long welded rails are available: new rails of standard length that have been laid along the existing jointed rails are welded into long rails on site and replaced with existing jointed rails, or rails welded to 150 - 200 m long at the factory can be
