The step up from 160km/h to 210km/h was a major breakthrough and required the development of several new technologies. It wasn't just a case of fitting a bigger motor and pushing the throttle a bit harder.
One of the biggest problems was oscillation in the suspension, which required a new type of dampener to be developed to prevent the train derailing at high speed. They also had to develop new pantographs to avoid destroying the overhead lines at high speed, and a new completely electronic and automatic signalling system as normal signals pass by too quickly for the driver to reliably see.
Over the 50 years of operation their safety record has been pretty much spotless, in a country that has regular major earthquakes. Punctuality is incredible too. In 2007 the average delay from the advertised schedule was 18 seconds, including delays due to natural disasters.
The current 320km/h is limited by concerns over noise. The trains are rated for about 380km/h, but they need to improve tunnels to prevent the noise created when the train emerges before they can go that fast.