On railway tracks, the sector where the rails or rails join together is called joint or joint. It constitutes the weak point of the track, especially from the dynamic point of view with the use of fish plates secured by bolts. The passage of the wheel through the joint causes a shock, the negative effects of which are to increase the tensile resistance, contribute to the longitudinal displacement of the rails, produce crushing of the ballast and impose bendings and deformations on the rail that can become permanent, this being the reason for its replacement by welding (RCS), which provides great advantages with the continuity of the track, and less maintenance, longer useful life of both the track and the sleepers, lower rolling resistance.

The basic function of the clevised joints is to allow the thermal expansion of the rails due to changes in temperature. In the case of welded joints, the expansion problem is accentuated, although from 12 m onwards, the friction produced by the fastenings absorbs all the expansion efforts, so the calculation can be carried out as a 12 m embedded beam.

There are various types of open joints (with fish plates), usually butt joints, in terms of their arrangement, they can be alternating or facing joints. The facing ones accentuate the gallop of the vehicles and the alternating ones, the oscillation with respect to the longitudinal axis, these being the ones used in Spain before being replaced by the welded ones. According to the type of support, they are classified as suspended (when they are located between a sleeper and a sleeper) or supported (when they are located on a sleeper). There are also temporary fish plates, combined to join different profiles, or electrically insulated, either with insulator or glued.

The minimum length of a track for continuous welded rail (RSC) to be considered is variable and depends on the type of rail, fastenings, sleepers, ballast and thermal regime of the area. In general, it is estimated to be 200 m, with rails welded in the plant by electric welding they vary between 120 and 800 m.