Earlier this month, TheTrainline confirmed plans to launch an IPO, saying it was hoping to raise around £75m, which would have valued it at around £500m.
The business, which made sales of £1.42bn in the financial year to March 2014, had brought in JP Morgan and Morgan Stanley to advise on the deal.
But today Exponent, its private equity owner, said it had sold the company.
Claire Gilmartin, TheTrainline's chief executive, said:
The continued growth in rail, shift to online sales of rail tickets, increasing popularity of mobile and eticketing, and the potential for expansion in Europe, all offer attractive opportunities for the company. We are looking forward to working with KKR to capitalise on these opportunities.