The Jinghu Line goes from Nanjing to Beijing but it's not a high speed rail. The only bullet train out of Beijing goes to Tianjin and then up to Shenyang. Nanjing is kind of in the middle of one of the fast rails. It goes east to Shanghai and it goes west out of Nanjing.
The high speed train Beijing to Shanghai opened to public months ago and it will stop at Nanjing for a while. It runs about 300 kph, costing about 640 rmb and 5 hrs to Shanghai (about 3.5 hrs to Nanjing). To purchase a ticket you need to show your ID.