It is an essay on killing and whether it can ever be justified. I don't know what Venerable Thanissaro would say about this train morality problem, but he does emphatically say that killing can *never* be justified and backs it up with quotations from the canon.
"Even if bandits were to carve you up savagely, limb by limb, with a two-handled saw, he among you who let his heart get angered even at that would not be doing my bidding. Even then you should train yourselves: 'Our minds will be unaffected and we will say no evil words. We will remain sympathetic, with a mind of good will, and with no inner hate. We will keep pervading these people with an awareness imbued with good will and, beginning with them, we will keep pervading the all-encompassing world with an awareness imbued with good will — abundant, expansive, immeasurable, free from hostility, free from ill will.' That's how you should train yourselves."
— MN 21
In the essay he also relates a story that might be illuminating of what the Buddha would say about this train morality problem:
When one of his monks went to an executioner and told the man to kill his victims compassionately, with one blow, rather than torturing them, the Buddha expelled the monk from the Sangha, on the grounds that even the recommendation to kill compassionately is still a recommendation to kill — something he would never condone.
The essay does not reference the sutta of the story above. I wonder if anyone recognizes it and can reference the sutta? I'd like to read it.