Simpson Thacher & Bartlett has picked up the lead role advising private equity group KKR on its purchase of travel company Trainline, as plans to float the online firm were shelved.

Simpson Thacher's London corporate partner, Alvaro Membrillera, is the lead partner on the deal.

Trainline was one of the first companies of the year to announce plans for an initial public offering (IPO), which was set to value the firm at more than £500m. It had hoped to raise around £75m. The business had hired JP Morgan and Morgan Stanley as it prepared for the float.

Travers Smith, which originally won the mandate to advise Trainline and its owner, Exponent, will act on the sale with a new team.

Private equity partner Lucie Cawood and corporate partner Ian Shawyer, both based in London, are handling the sale.

Exponent bought the business for £160m from a consortium that included Virgin, Stagecoach and National Express in 2006.

Previously, Trainline had enlisted Travers Smith London corporate head Spencer Summerfield alongside corporate partner Adrian West and US securities partner Charles Casassa to advise on the IPO deal.

The banks charged with leading the IPO had previously enlisted Herbert Smith Freehills corporate partner Chris Haynes and the firm's global head of capital markets, Steve Thierbach.

Neither Trainline nor Exponent provided an explanation as to why the firm had shelved the IPO deal.

However, dual tract transactions, where a company keeps its options open to either launch an IPO or sell by way of private sale, are common.