MEXICO CITY—A preliminary report by experts into the collapse of a Mexico City elevated subway line that killed 26 people placed much of the blame Wednesday on poor welds in studs that joined steel support beams to a concrete layer supporting the track bed. The city government hired Norwegian certification firm DNV to study the possible causes of the May 3 accident, in which a span of the elevated line buckled to the ground, dragging down two subway cars. The report also said there were apparently not enough studs, and the concrete poured over them may have been defective; the welds between stretches of steel beams also appear to have been badly done. “The studs showed deficiencies in the welding process,” the report states. The report centered on photos and physical inspection that showed that metal studs welded to the top of steel support beams had broken or sheered off …