Someone determined on jumping in front of a train would have had these thoughts for a long while and likely received support from various places already. A "small talk" with some random woman off the bench isn't going to suddenly make him not suicidal.
That's not actually what studies show. Yes, people experiencing depression may have suicidal thoughts over a long period of time but the actual shift from suicide ideation to taking action is actually normally a terrible quick impulse - a study a while ago working with survivors found that the average length of time between deciding to make an attempt and the attempt is 12 minutes.
That's why so many seemingly small interventions can have such a big impact- like making it harder to buy large quantities of paracetamol for example, may mean that someone does not take an overdose because in the time it takes them to walk to three different shops, that moment of crisis has passed. It is also rare (sadly not that rare) for someone who survives an attempt to try again which is why psych wards usually discharge patients quite soon after an attempt.
Honestly, from my experience as a survivor of attempted suicide and someone who does attend support groups for such, I think the majority of suicides are sadly very preventable and I think the line 'there is nothing you could have done to prevent it' is a comforting lie told to make it easier for grieving family members. Of course, it isn't anyone's fault when someone takes their own life, and it's bad luck that puts someone alone and within reach of means when at crisis point but I do think the narrative that suicides are super determined people who no one could stop from taking their own lives is, 90% of the time, not accurate.