Union Pacific is pulling rail cars from storage for the first time in four years to haul lumber for homebuilders, a welcome source of strength for an industry that’s bucking the effects of a slowing economy. North American rail carloads of lumber and wood products this year through July 28 were 10 percent higher than the same period in 2011, according to the Association of American Railroads. That compares with a 1.4 percent drop in total carloads. New home construction climbed in June to the highest in almost four years.