National Express's main shareholder, Spain's Cosmen family, today launched a bid for the troubled rail and bus group in conjunction with CVC, the private equity group.In a stock exchange statement, CVC said it and the Cosmen family had made an indicative cash bid.The Cosmen's own 18.5% of National Express and sent a letter to the board three days ago, according to the FT this morning. Any deal would value the debt-laden transport group at more than £500m, the paper said.National Express revealed it had received a second takeover approach late on Wednesday soon after rival FirstGroup dropped its takeover plan. National Express did not disclose the identity of the new bidder and said its intentions were "not yet known".Today's reports suggested the board of National Express is looking for a bid of at least 400p a share, valuing National Express at nearly £620m, and well above the Cosmen's valuation.Market rumours yesterday suggested bus and train group Stagecoach and Go-Ahead may be mulling offers according, but analysts were sceptical given National Express's problems. The firm is struggling under debts of £1.2bn. It recently walked away from the franchise to run the East Coast Main Line after the two sides failed to renegotiate the terms of the franchise. The government has also threatened to take away its other two rail franchises, though it may not legally be able to.The Cosmen family first became national Express shareholders when they sold it coach firm Alsa to National Express for £149m and a 10% stake. Jorge Cosmen is deputy chairman.