That is standard practice in many places. I posted examples of such used in India elsewhere.
How common is it? Seems in the US a specialized crew de-stresses the rail to accommodate a certain temperature range before welding. But I only know that from Roaming Railfan videos. There were also two types of welding, but that's irrelevant.
I don't think anyone's mentioned the tremendous other kind of stress the railroads are under since the Norfolk Southern derailment. At least in my area temperatures are not historically high, it's the storms and flooding that have increased, as well as warmer winters. (Afternoon thunderstorms are actually a return to what farmers and gardeners expected before summer droughts began in the late 1970s, but they are lots more intense now.)
When the chief of NS is dragged before Congress and grilled on labor practices and safety, with big financial hits at stake, there are going to be slow orders at 90 degrees all over. I'm
sympathetic to NS biased towards NS compared to other freights, even if it moved HQ from Norfolk to Atlanta (better airport!). The run between NFK and PTB is excellent rail, then it gets progressively jumpier from RVR to NYP.
Someone mentioned welded rail is better for the equipment. The NYC subway's rebuilding is partly to allow less heavy duty equipment and track, de-escalating the fight between them, with the rider suffering.
(Also, mainline rail may be in getting worse? Edit: and Congress should raise standards for staffing etc. that all the freights have to follow.)