Trains are faster than planes for short-ish inter-city journeys in many other places because (i) you can use public transport to get to the stations, (ii) you avoid the time spent on check-in and security check and (iii) trains are more frequent than planes to the same destination.

None of these is the case in Beijing. I don't know about other cities. I've taken inter-city trains from Beijing a few times. Each time, I've found the experience of getting to the station, and finding the right spot, more arduous than for international flights.