U.S. private-equity firm KKR & Co. agreed to buy online U.K. rail ticket retailer Thetrainline.com from Exponent Private Equity for an undisclosed sum.

Trainline, which Exponent bought from Virgin Group Ltd., Stagecoach Group Plc and National Express Group Plc for 163 million pounds ($247 million) in 2006, is the largest provider of rail tickets in the U.K., London-based Exponent said in an e- mailed statement on Thursday.

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