You can book a Bejing to Lhasa train ticket at the railway station. To book a ticket, you need to bring with you your original passport as well as a photocopy of your Tibet Travel Permit. You can also contact Great Tibet Tour and we will be very much willing to book you a ticket on your preferred travel date. At the West Railway Station of Beijing, you should proceed to counter 16 at the north square's east ticket hall. There, you will find an English-speaking employee. Make sure that you get the right one when you book a ticket.
You can also book a ticket on 12306.cn which is the official ticket-booking website. This ticket-booking website is operated by China Railway Engineering Corporation. Through this website, you can accurately and immediately book a train ticket to Tibet. Before you can book, however, you need to register and log in. After you have booked a ticket, you can then check the confirmation page and then pay up. You can pay via Chinese Banks, Alipay, or UnionPay. The problem is that you need to know how to read Chinese before you can book a ticket on this website because the website used Chinese characters.