"It took me a second to figure out what was happening ... I saw the lights in the distance. My mind was to alert the train," Abbasi said.
This is a tragic story about an unnecessary loss of life. It's also a fascinating look at how people react. The man who took this picture has responded to questions about why he took a picture by instead focusing on how upsetting he found the behaviour of people closer to Mr. Han than he was. Why did he feel this necessary? Probably because he's got some internal cognitive dissonance between what he knows would have been "moral" behaviour (help Han up) vs. what he actually did.
Hindsight is a funny thing; it forces us to offer logical rationalization for reactive behaviour that is more gut instinct than logical reasoning. Reactive behaviour relies on pre-programming, which happens much more quickly than abstract thought. It shouldn't really be a surprise that a photographer would respond to an uncomfortable situation by turning to comfortable, established behaviour - like taking pictures.
Police, soldiers, fire fighters, even CPR students all go through test-case scenarios that help build familiarity with the sorts of stressful scenarios they could encounter in their jobs; consider it a kind of emotional inoculation. While some people are more naturally inclined to react well under pressure, everyone can learn to handle prescribed situations better. This principle underlies everything from cognitive behaivoural therapy to improv class.
There will be a lot of focus on what didn't happen in this situation; it's an instinctual approach to take. A more useful one, though, might be to think about what could be learned to prevent such tragedies in the future.