The following is an account by a Hong Kong resident who was embroiled in the incident.
On that day I was with my girlfriend at Hung Hom Station. All the seats in the cabin were taken. On one side was an adult and three children, on the other side were three adults with their luggage. Later, I saw the mother pouring some noodle snack into the hands of one kid, while the other two girls were holding some candies. The kid with the noodle snack spilt some on his clothes and on the floor. I used my limited Mandarin [standard Chinese of mainland China], pointed at the “No eating allowed” sign in the cabin, and told them: ‘Excuse me, you can't eat here’. What happened next was a round of defense. The mom said that only the kids were eating, that she was about to take it away, and that it was none of my business… I told her that I saw her eating as well… I knew I could not change their attitude and was about to shut up, but then her friends sitting opposite to her, sneered at me: “Their Mandarin is so lousy.” I got so angry and talked back in Cantonese: ‘My Mandarin is lousy! This is Hong Kong, you should speak in Cantonese!’ They kept on defending themselves in loud voices. A man in the cabin stood up by my side: “Shut up! It's so noisy here! Just keep on eating OK? How could you yell like this when you have done wrong? Should I call the railway staff?” As they continued to speak loudly, the man pushed the button to call staff when the train reached the next station. And I waved the railway staff over to take care of the situation. What happened next, you can see on the YouTube video.