can you tell me the shortest train ride from Xian to Hong Kong?

There is no direct train from Xian to Hong Kong.
You can take a bullet train from Xian North to Shenzhen North first. It will take 9.5 hours on the way. Then take Longhua Subway Line to Fu Tian Checkpoint. This will take around 40 mins.Then pass the custom and arrive in Hong Kong.
It will take 10 hours on the way