Airlines are asking Congress to amend the federal law that governs airline labor relations to make it harder for unions to strike. The Air Transport Association, the industry’s largest lobbying group, is urging law-makers to revise the Railway Labor Act of 1926, the federal law that regulates the industry’s labor relations. Airlines, which have been struggling to regain profitability, say labor expenses are their biggest costs. If the ability to strike is limited by the federal government, it would