Answered by ZUPAN | Jun. 03, 2019 14:55
No, your itinerary is not eligible for 144 hours visa free transit because you cannot use a train or any other mean of transport between Hong Kong and Shenzhen. Anyway, you have two options that you could possibly consider:
1. After your arrival in Hong Kong, you can take a local transport, preferably through Lo Wu (Luohu) border crossing where you can obtain Shenzhen 5 day visa on arrival. After your stay in Shenzhen you are allowed to take a plane from Shenzhen to US. This option is the simplest one, but it has two drawbacks. The first one is the price of Shenzhen VOA for US citizens that amounts to 120 USD (payable in CNY), equivalent to 10 years multiple entry visa for China if obtained in US. The second one is that you are limited in your movements to Shenzhen Special Economic Zone only.
2. In order to become eligible for 144 hours visa free transit, you would have to take a short flight from Hong Kong to Guangzhou. Upon arrival you will be granted 6 days free entry permit and the best detail is that you are allowed to travel throughout Guangdong province. Of course, you are allowed to travel to Shenzhen because it is also within Guangdong province. You can travel from Guangzhou to Shenzhen easily by train. After your stay in Guangdong province, you can take a plane from Shenzhen to US.
Now, it's up to you to decide.