To get to Tibet from mainland China by train, you need to decide which city you are going to take the train firstly. Currently, there are 7 cities including Beijing, Guangzhou, Shanghai, Chengdu, Chongqing, Xining and Lanzhou connected Tibet with direct train.
Then you need to book a Tibet tour as well as the train ticket to Lhasa from a local Tibet Travel agency, deal with the related permits and visas before your departure.
Lastly, board the train to Tibet from mainland China and enjoy the breathtaking veiw along the Qinghai Tibet Railway.
You can find direct train to Tibet from the cities of Chengdu, Chongqing, Guangzhou, Shanghai, Beijing, Xian, Xining Lanzhou. Among them, I would suggest you to take the train to Tibet from Xining. The reasons are as follows:
1. The train journey from Xining to Lhasa only takes 20 hours, you don't need to spend too much time on the narrow space of the Qinghai Tibet Train.
2. The highlights of your train journey to Tibet lies in the section of Xining - Lhasa part which includes Qinghai Lake, Keke Xili Grass Land, etc.
3. All of the trains to Tibet will pass by Xining Station, you have more choices on the trains to Tibet.