Because rails are made from steel, they expand as they get hotter, and can start to curve this is known as 'buckling'. Most of the network can operate when track temperatures heat up to 46°C – roughly equivalent to air temperature of around 30°C – but rails have been recorded at temperatures as high as 51°C.

Rail buckling, often referred to as a "sun kink," is primarily caused by extreme thermal expansion in the steel rails during periods of intense heat. Steel expands as it gets hotter, and because modern railway tracks use "Continuously Welded Rail" (CWR)—where miles of track are joined together without gaps—there is no room for the metal to grow lengthwise. When the internal stress caused by this expansion exceeds the ability of the ballast (the rocks) and the "sleepers" (ties) to hold the track in place, the rail suddenly and violently "buckles" or bends to the side to relieve the pressure. Other contributing factors include inadequate ballast support, improperly installed rail anchors, or if the "neutral temperature" (the temperature at which the rail was originally laid and tensioned) was too low for the current climate. In 2026, rail operators use "heat slow orders," requiring trains to travel at reduced speeds when temperatures spike, to prevent the vibrations of a passing train from triggering a buckle in a stressed rail. Rail buckling is a major safety hazard, as it can easily lead to a derailment if a train encounters a kinked section of track at high speed.