Expanded capacity for freight trains would make a lot of sense - maybe not so much for China, because it´s much cheaper to transport stuff on ships, but for all the landlocked countries between China and Europe.
High-speed trains? No.
A) You can´t run high-speed passenger trains and freight trains at the same time. The pressure wave of an high-speed train would take a freight train apart.
B) Lines for high-speed AND freight trains are extremely expensive to build. Freight trains can´t cope with uphill gradients and high-speed trains can´t cope with tight bends. If you need to cross mountains (and there are A LOT between China and Europe) you´ll need a lot of pricey tunnels and bridges that way. This will make the trains expensive to operate and the potential passengers in the middle Asian countries have no deep pockets to pay for expensive train tickets.
That sounds like another silly mega-project like the three gorges dam...