It is only one rail line and some roads... it takes a couple of B2s to destroy the link. The US doesn't have to hit china itself (to avoid any nuclear escalation). It will hit the silk road in the secondary countries where it passes (eg. Kazakhstan). Siberia is huge and the Russian - China link is very inefficient.

Also, China is in bad terms with almost every other neighbor it has (territory disputes with almost every one).. and they will be glad to see it humbled a notch or two and will not help.