China is also extending the rail networks in Tibet, and possibly the most serious threat to China's internal stability came from the riots in Xinjiang province last year. Tensions in both Tibet and Xinjiang are still simmering, largely because the non-Han residents feel left out of the equation every time jobs and wealth opportunities appear in their areas.
There is also a credible threat (well, depending on who you choose to get your news from, but hey)from the resident Chinese Muslim population, many of whom live in Xinjiang province, and who identify more with Pakistan, Khazakhstan and Turkmenistan than they do with the Middle Kingdom.
I'm wondering if the routing of the railway line has something to do with this, to give idle hands some work and some access to cash in these areas, and to extend influence into neighbouring countries so that if something happens in the future, China has some real leveraging tools to get things going in the direction that they like. Hell, it could even be used as a reason for a pre-emptive invasion: " Well , we had to go in, it was clear for all to see that country X was not able to safeguard a vital international asset that all of Asia and Europe uses. But no need to thank us for the invasion, we were only doing what any country would have done."