Yes, boiling water is available on all the trains to Tibet. Usually, the water is considered safe to drink when it is boiled at 100 ℃. Yet in high elevation, the water boils are a little lower temperature. Here in the Tibet train, water boils at 70℃.
On the Tibet train, each cart has its water boiler, and you can get it for free of cost. There is a water boiler between two carts. Water is available 24 hours, but during peak hours in the day, you will have to wait sometime to get the water to reach the boiling temperature.
While getting the water, you might want to be a little cautious about the moving train. You will have to bring your mug and thermos.
There is hot water available in the sink area. But, it is only for washing things, not for drinking. Therefore do not drink from that hot water tap in the sink area. For it is undrinkable.
Since boiled water is plenty available on the train, it is recommended for travellers to instant food like noodles, oatmeal, tea, coffee, and anything else that requires you to add just hot water before eating it. You can also choose to buy Chinese Instant noodles abroad.