Good point. If China decides to attack India, India could conceivably attack the Qinghai-Tibet railway. However, the truth cuts both ways--if India attacked into China, it could sustain a 10-15 km incursion at most, since infrastructure leading up to the Indian side of the Himalayan foothills is pretty bad, and China could likewise hit the supply lines.
To be honest, a Sino-Indian war would be prolonged, bloody, and most ineffective on both sides. Moving men up mountains is hard. Moving equipment and supplies is even harder. Hence, making advances in mountainous terrain is difficult for any force, especially when the elevation is so high that it gives all the soldiers hypoxia and frostbite. Most of the units involved will be purely light infantry, but China does have a decent attack chopper with a heavy payload and high-altitude performance (the WZ-10), so fire-support from the Chinese side is not likely to be an issue as those choppers can substitute for artillery.
To use them, though, China will need to win air superiority. That's going to be difficult. Air parity is the likeliest outcome, unless Pakistan decides to join in the fun and force the IAF to divert its assets. If the assets are diverted, of course, then the PLAAF will have pretty much free reign over the skies reaching 100 to 200km into India until Pakistan is removed from the equation (which could take anywhere between 5 to 15 days). This means it'll be a light-infantry vs light-infantry skirmish on the ground, but the IA is going to have no ability to move because any movement will be interdicted by the PLAAF and they will be outgunned because every skirmish will have CAS from the Chinese side but none from the Indian side. This combination of factors may produce substantial advances and open holes large enough to punch mechanized forces through, but it's still unlikely.
Ergo, unless Pakistan is fighting India, any Sino-Indian conflict is going to be bloody and inconclusive. But if Pakistan is also fighting India at the same time, the IA is going to have its hands very, very full. Chinese mechanized formations could conceivably punch all the way to the border of Bangladesh, and split the eastern provinces off from the rest of India.