That said, if one accepts the premise, and accepts that it's really necessary to construct the tunnel for whatever reason, and worth the certain loss of lives, then yes, it's most practical to crush the guy and move on with the project.

I would question the practicality, given that it has rather significant externalities with respect to the effect on all the construction workers on the project. It seems like a situation where inefficiency could be useful for cooperation between the workers. ("Leave no man behind!")

Moral considerations aside, if it's going to kill my mother or something I would certainly not press it, not even if it saves five other lives.

If it's someone I care about I'm not just going to not press it I'm going to destroy the button device and all others like it that I can find!