By David Shepardson WASHINGTON (Reuters) -A labor union representing about 4,900 rail workers said on Saturday that members narrowly ratified a tentative contract agreement with freight railroads in the United States. The union representing locomotive machinists, roadway mechanics, and facility maintenance personnel is the seventh of 12 to approve the deal, while two unions previously voted to reject the national deal announced in mid-September. The International Association of Machinists and Aerospace Workers (IAM) District 19 said 59% of the membership voted and it was approved by 52% of vot…