I think you may be confused. There's the PRD Mair that is coming up to 600 km on the mainline rail system (not HSR).
Then there are the metro systems for each city that add up to 815km as of 2018, but will likely surpass 1000km this year - or halfway stage in their expansions to connect up:
Guangzhou -Foshan (China's first intra-city metro that connected up in 2010. Currently the world's third longest after Beijing and Shanghai at 487km - but possibly world's largest now as it had 325km u/c - and 3rd highest ridership)
www.travelchinaguide.com,
www.chinadaily.com.cn
Shenzhen (world's 12th longest - 294km -and 6th highest ridership, or about 200 million more journeys than NYC).
www.mtr.com.hk
It's currently 'connected' to HK's network but still with the border control.
https://www.reddit.com/r/MapPorn/com...hen_metro_map/
and Dongguan, 38km - only 1 line but 3 other u/c. Next year it will connect up with Shenzhen's
http://en.gde.cc/Audience/info.aspx?...anZhongRuChang,
https://www.thatsmags.com/shenzhen/p...n-and-dongguan
To reiterate those systems are already connected up by the PRD MAIR (read: mainine rail system), but are working to connect themselves up without it also.