The bearings in trains are actually pretty complicated to build. They use slanted roller bearings, not just regular old ball bearings. 10 companies in the world make like 70% of all rail bearings, and 5 of those are in Japan. Plus, only a few countries produce the two steel alloys that you make these bearings out of.

China does produce around 20% of the bearings used in trains, but pretty much all of it gets used for domestic purposes and they still have to import due to their massive rail expansion projects

100% this isn’t a lost technology, it “just” needs some machines built

The ball bearings are made in Germany with a very specific level of precision that China can definitely replicate given how they rapidly caught up to the rest of the world in the past 30 years economically and technologically. (Hint was done primarily through corporate espionage and theft)