Does 144 visa free entry apply to Hong Kong visitors?

No, you cannot do that. Hong Kong is eligible departing point if you plan to use 144 hour visa free transit, but you cannot enter Guangdong province (Shenzhen, Guangzhou, Zhuhai...) by train from Hong Kong. Currently the only eligible way to enter Guangdong province under 144 hour visa free transit is by air. In other words you would have to take a flight from Hong Kong to Guangzhou and after your stay that allows you to travel, stay and explore entire Guangdong province, you can take a direct train from either Shenzhen or Guangzhou to Hong Kong West Kowloon train station or a ferry from Shenzhen to Hong Kong or Macau or a ferry from Zhuhai to Hong Kong... Exits are more liberal than entries. Also, if you decide to enjoy this option you should have in mind that you will have to buy your train or ferry ticket even before you depart toward China, to print it out and to take exactly that train/ferry you have bought your ticket for.

Alternatively, you can visit Shenzhen by applying and obtaining a 5 days Shenzhen visa on arrival at the designated border crossings for 130 CNY or 3 days Zhuhai visa on arrival at designated border crossings for the same price. Please, be careful because you cannot travel directly from Shenzhen to Zhuhai under these visa on arrival schemes. You have to exit Shenzhen to Hong Kong or Macau and then enter Zhuhai or vice versa. Hope this helps.