Yes, the Chengdu-Lhase line is difficult to build, but it has very high priority in Chinas plan and development.
I understand, a HSR link to every provincial capital is felt as important for providing similar services to people, everywhere in the country.
Equally, If BRI is to take off, and link to the Middle East and Africa, the shortest and less politically entangled and most economical route is directly through Pakistan.
This route also mitigates some of the risk of disruption of supplies vis a vi the Malacca Dilemma.
$30 billion invested in a BOT model (in the case of Pakistan railways mainline 1, primarily as a freight line and mostly parallel, not replacing, the existing line would leave the existing track for commuter rail and another revenue stream to help make the investment sustainable). An additional $10 Billion would be a joint venture between the central Asian countries, Afghanistan and Pakistan.
China portion of the railway would cost less then $10 Billion, but to complete the Pakistan-China line, Pakistan would be borrowing $15 billion specifically for this line and employing Chinese companies, especially because of their experience and expertise. It’s a win for Chinese business upfront and for China long term.
For China, this line could even form the basis for a line that continues onwards to Askai Chin and continues along the length of the border with India (with routes to go into parts of Nepal to generate economic activity and raise GDP and business opportunities in Tibet). This could be very useful for PLA logistics, especially in moving men and materiel to and from posts along the border.
A route from Xigaze to Burang is something the Indians think China may one day build and are keeping an eye on.
This route could be connected to Kashgar via the CPEC rail route by a spur that goes to Aksai Chin and eventually onwards to Burang
For military logistics it would be a valuable route but if the near by area had mineral resources, Increased access to mining sites in Tibet could make this route sustainable.