Entry by Train, exit from HKG

Hi, I am planning a train trip to China, entry will be via the train from Ulan Bator, I have a reservation confirmation for that, exit will be from HKG via Shenzhen, I plan to take public transport (metro) to and from the border there, with me planning to leave Hong Kong 7 days after I leave mainland China. Do the One way train ticket and the flight out of Hong Kong suffice for the "round trip plane ticket" tourist visa requirement.