There is also a difference between doing nothing and doing things in a way which might be less effective than maximum effort but which would be safer for yourself.

e.g. if I'm alone and unarmed, and I see someone stopped involuntarily by the side of the road and relatively safe, in a bad area, in a normal-to-sketchy car, etc., I will probably call the police to report it, rather than stopping myself. If I see an elderly person in a rich area in a non-threatening car with a flat tire, I'll possibly stop and offer to help, or at least put better flares or whatever behind his car and make sure he's ok for water/health while waiting. If I see an actual accident in realtime, or the aftermath of an obvious accident, with likely trauma to passengers, I'll almost always stop (unless police/paramedics are already there).

Similarly, if someone was standing on the tracks with an approaching train, I'd verbally command him how to GTFO, and possibly belt/bag-as-rope, but particularly if it were an adult stranger who is potentially suicidal or otherwise unlikely to cooperate, I'd be instinctively reluctant to get in front of the train myself. Someone who accidentally fell would scare me less, but would also be more likely to be able to get out on his own or with assistance from the platform.

But the time to make all these decisions is well in advance of the incident; what level of comfort you have with different kinds of threats, skills in dealing with things, etc. I'd probably be more comfortable with water rescues (since I actually trained for this as a dive master) than most people, but less comfortable dealing with crazy/mentally unstable people, and more comfortable with end-a-hostile-threat vs. prevent-self-harm.