Yes, the trolley is the instrument of death in the trolley case.
And nope, the surgeon’s scalpel that excises the mother’s diseased tissue is not the instrument of the child’s death as Fr. Tad explains:
Importantly, the surgeon is choosing to act on the tube (a part of the mother’s body) rather than directly on the child.
Again o_mlly is using Fr. Tad to support that the surgeon acting on the tube is not direct in such a way that Fr. Tad may very well also say that the bystander acts on the trolley, not on the man on the tracks. If both problems are analyzed according to the same definitions and standards, both problems will yield the same result.