China Railway Construction Corp., one of the country’s biggest infrastructure builders, plans to seek as much as $800 million in a spinoff of its tunneling machinery business, people with knowledge of the matter said. China Railway Construction Heavy Industry Co., known as CRCHI, is working with advisers and plans to file a listing application with the Hong Kong stock exchange as early as this quarter, the people said. The company aims to start trading this year, said the people, who asked not to be identified because the information is private.