Wagon train morality here is referring to the duty we offer to each other in a crisis, but this hints at something larger.
See, I think that we owe this duty in ethical terms.
But if you reject this idea of ethics, I mean, who cares about morals anyway?
That’s just your opinion. Right?
But we are also human in, the physiological sense, that desperation, and pain, and hunger can actually turn off the rational parts of our brain.
In a larger passage, Didion speaks about the Donner Party here.
You can read about it in the link above, if you want your stomach to turn like that.
If we have been taught to keep our promises—if, in the simplest terms, our upbringing is good enough—we stay with the body, or have bad dreams. I am talking, of course, about the kind of social code that is sometimes called, usually pejoratively, “wagon-train morality.” In fact that is precisely what it is. For better or worse, we are what we learned as children: my own childhood was illuminated by graphic litanies of the grief awaiting those who failed in their loyalties to each other.
-Joan Didion, Slouching Towards Bethlehem (Amazon)