Hanoi to Nanning / Hohhot to Ulan Bator by train : Visa Problem

Hello. We want to travel from Vietnam to China by train (Hanoi to Nanning) and leave China to Mongolia by train (Hohhot to Ulan Bator). To apply for a China Visa in Switzerland, we need to show in and out tickets. But tickets are only available 18 days in advance, how can solve this Problem?