I was standing on a train today, holding the handrail.
Then, a girl got into the train, and stand in front of me, nearly face to face.
I found her blinking very fast, yet I didn't talk to her.
I want to ask, can it be considered as an IOI? Thanks.

Who cares. You don't need IOIs to approach. Maybe she had dry eyes. When I put my contacts in the morning, my eyes get very dry and I blink more...

So anyhow, what prevented you from approaching her? What excuse did you tell your ego not to approach her?

_________________
Drink me, make me feel real
Wet your beak in the stream
Game we're playing is life
Love is a two way dream

No this is not considered an IOI anywhwere I've seen.

Her contacts could have been dry and were bothering here. She could have had something in her eye that her eyes were trying to flush out. Allergies could have been bothering her. There are too many factors that go into why people blink too consider this a solid IOI.

Were there any other indicators of interest that you saw from her? Remember a single IOI by itself isn't that good. For example a girl touching her hair by itself doesn't mean much. A girl you are talking to giving you good eye contact, touching her hair, smiling, and reinitiating conversation with you is better if you get what I'm saying here.

I hope this threat isn't a joke.

If you want to talk to the girl then do it. You don't really know for sure if she's into you until you talk to her. I'm still surprised to this very day about the girls who turn out to be both really friend and then ones who are super unfriendly - I can rarely tell upon first glance how they'll act.