You realize trains have times to arrive at certain places, right? So we can see the distance between two places and the time it takes to get there and therefore know how fast the train is going.
The fastest train in the United States is the Acela express (this is the only high speed rail the US has) travelling between NY and DC. NY and DC are 364 kilometres apart. It takes 2 hours and 45 minutes to get there at 132.3 km/h. The Beijing-Shanghai high-speed railway, by comparison, has to travel a distance of 1,302 km, and averages 291.9 km/h across this journey, making a trip of 4 hours and 18 minutes.
To fake these numbers, the Chinese government would have to fake their entire transportation schedule to make it look like the trains are arriving faster than they are (which means the train schedule would be unusable) and get essentially their entire population and anyone who visits there to lie about the time it took for the train to get there.
If the numbers were faked, we would know. To hide such a thing would require the most coordinated conspiracy in the history of mankind.