(Updates with official statement) By Greg Roumeliotis May 13 (Reuters) - U.S. railway operator Kansas City Southern said on Thursday that it had accepted Canadian National Railway Co's $33.6 billion acquisition offer, upending a $29 billion deal with its competitor Canadian Pacific Railway Ltd. The development, first reported by Reuters, gives Canadian Pacific five business days to make a new offer for Kansas City Southern. Were Canadian Pacific to table a new offer, a bidding war could ensue.