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.

The decision to sell to New York-based KKR comes just two weeks after Exponent announced plans for an initial public offering in London in a transaction that would have valued the company at about 500 million pounds.

“The continued growth in rail, shift to online sales of rail tickets, increasing popularity of mobile and e-ticketing, and the potential for expansion in Europe, all offer attractive opportunities for the company,” Clare Gilmartin, chief executive officer of Trainline, said in the statement. “We are looking forward to working with KKR to capitalize on these opportunities.”