Continuous welded rail and why it doesn't buckle or "sun kink."

Short answer: it's laid under just enough tension that it doesn't expand when it's warm out, and fixed in position so tightly that it doesn't move. 

https://en.wikipedia.org/wiki/Track_(rail_transport)

"If not restrained, rails would lengthen in hot weather and shrink in cold weather. To provide this restraint, the rail is prevented from moving in relation to the sleeper by use of clips or anchors. Attention needs to be paid to compacting the ballast effectively, including under, between, and at the ends of the sleepers, to prevent the sleepers from moving. Anchors are more common for wooden sleepers, whereas most concrete or steel sleepers are fastened to the rail by special clips that resist longitudinal movement of the rail. There is no theoretical limit to how long a welded rail can be. However, if longitudinal and lateral restraint are insufficient, the track could become distorted in hot weather and cause a derailment. Distortion due to heat expansion is known in North America as sun kink, and elsewhere as buckling. In extreme hot weather special inspections are required to monitor sections of track known to be problematic. In North American practice extreme temperature conditions will trigger slow orders to allow for crews to react to buckling or "sun kinks" if encountered."

After new segments of rail are laid, or defective rails replaced (welded-in), the rails can be artificially stressed if the temperature of the rail during laying is cooler than what is desired. The stressing processinvolves either heating the rails, causing them to expand,^[16] or stretching the rails with hydraulicequipment. They are then fastened (clipped) to the sleepers in their expanded form. This process ensures that the rail will not expand much further in subsequent hot weather. In cold weather the rails try to contract, but because they are firmly fastened, cannot do so. In effect, stressed rails are a bit like a piece of stretched elastic firmly fastened down.

CWR is laid (including fastening) at a temperature roughly midway between the extremes experienced at that location, known as the "rail neutral temperature." This installation procedure is intended to prevent tracks from buckling in summer heat or pulling apart in winter cold. In North America, because broken rails (known as a pull-apart) are typically detected by interruption of the current in the signaling system, they are seen as less of a potential hazard than undetected heat kinks.