BE IJING, April 29(Reuters) - China's top two trainmakers have been in discussions with Bombardier Inc about possibly buying a controlling stake in the Canadian company's railway unit, two sources with direct knowledge of the matter said. China CNR Corp Ltd and state-owned CSR Corp Ltd , now in the process of merging to create the world's biggest railway company, are exploring a stake purchase of the Bombardier unit, complementing China's plan to sell its high-speed rail technology abroad, the sources said. But discussions can not move forward until after the Chinese trainmakers complete a planned $26-billion merger next month, said a person close to one of the Chinese companies.