Due to the shape of the rail, there is an irregular distribution of material over the cross-section. The electric current applied during flash-butt welding heats the rail up to melting temperature. 

Cooling behaviour of the rail

Consequently, the rail does not cool down uniformly over the entire cross-section. The rail foot and the rail web cool down first. As the largest proportion of material is in the rail head, it cools down far more slowly. The natural consequence is a negative bending of the rail in the area of the welded joint.

This result would have very negative effects on the track geometry and the operational safety. For this reason, a slight upward inclination of the two rail ends is performed, adapted to the respective rail profile, so that an ideal geometry of the rail is achieved when the weld has cooled down.