Shares in Trainline slid after its former private equity owner and other investors sold £285 million worth of shares in the business.
KKR, which had owned a majority stake in the company until it was floated on the stock market in June, was part of the group which sold 65.5 million shares at a price of 435p each.
Despite the major share sale, KKR will retain 59.6 million shares in the business, representing a 12% stake in the business.
KKR purchased the bookings business in 2015 for £500 million, before taking it public with a valuation close to £2 billion on its first day of trading.
Trainline delivered £129 million in revenues for the six months to August 2019, up 29% on the same period last year.
Other key shareholders including funds managed by Index Ventures, Ares and Alven Capital also halved their stakes in the business.
The US private equity giant also confirmed that, as a consequence of reducing its stake, it will lose one of its directors on the company board.
It confirmed that Franziska Kayser will no longer be a director on the board, with Philipp Freise becoming KKR’s sole representative.
Shares in the company slid 5.1% to 443p in early trading on Friday.