In university I wrote a paper on the bystander phenomenon, specifically as to why a group of onlookers to a crime or accident don't assist. There's multiple factors, like fear for their own safety, but mainly it's because everyone thinks someone else will help.
If I were in this situation - watching this man try to pull himself to safety - and I was the ONLY person nearby I admit that I'd feel a lot more responsibility to help him than if I were in a group of people. Does that make sense? I guess I'd quickly feel like the onus was on me and me alone to do whatever I could to help get him off the tracks knowing full well his life depended on my actions.
Yeah I heard something similar to this before too, i think it make a lot of sense. And I liked your post....not because I particularly like that we humans behave this way, just that I think it makes sense and is totally accurate.