You appear to be asking for categorical answers to hypothetical questions.
Depending on the circumstances and many real-time factors, one may or may not jump onto the tracks.
Trying to put ourselves into the mind of an arahant, so to speak, is dubious and in the end, impossible. Some folks try to imagine this.
But we can only work with where we’re at.
It seems the best we can do is embrace and manifest the precepts and teachings as best we can, knowing the responses will change for the better as our practice deepens.
The less self-view, greed, anger, and ignorance in the mind, the more beneficial our actions and choices will be, being grounded more in the Brahmaviharas and wisdom than in the defilements.
Within that clarity and openness of heart, we may jump on the tracks. Or not.
The Simile of the Saw MN21 example does not say one should get in line to be sawed next or to necessarily take another’s place. The Buddha said one should not allow such an event to produce a mind filled with anger and hatred.
"Even if low-down bandits were to sever you limb from limb with a two-handled saw, anyone who had a malevolent thought on that account would not be following my instructions. If that happens, you should train like this: ‘Our minds will remain unaffected. We will blurt out no bad words. We will remain full of compassion, with a heart of love and no secret hate. We will meditate spreading a heart of love to that person. And with them as a basis, we will meditate spreading a heart full of love to everyone in the world—abundant, expansive, limitless, free of enmity and ill will.’ That’s how you should train. "