Why does the Qinghai-Tibet Railway only have one track?

Details
Title | Why does the Qinghai-Tibet Railway only have one track? |
Author | China.org.cn |
Duration | 5:17 |
File Format | MP3 / MP4 |
Original URL | https://youtube.com/watch?v=zoqPxNyYONE |
Description
The Qinghai-Tibet Railway is the world’s highest and longest plateau railway. If traveling to Lhasa by train, you will discover that the most important part of the rail is a single-track railway: but why?