China currently has one of the largest public high speed rail systems and continues to invest in its expansion. This is a complement to that to help establish more reliable trade routes with its neighbors while simultaneously boosting their involvement in those economies and their dependence on China. Much better use of resources than genocide and regional destabilization through war (like some other imperialist nations) if you ask me.

Cars area huge export market and are the foundation of a number of economies (Germany, Japan, and soon China).

They bring prosperity to the countries that manufacture them.

Also, frankly, a lot of Central Asia is really sparsely populated. Public infrastructure requires a certain density.