Planning for a second express rail link in southern China’s most vibrant urban cluster is being revved up to streamline travel between the tech hub of Shenzhen and the commercial powerhouse of Guangzhou, while also benefiting Hong Kong.Construction is now in full swing after Shenzhen broke ground on the southern end of the future high-speed rail artery in June. The project begins with a new station at Terminal 1 of the city’s airport, with space reserved for a future cross-border extension into...