A train’s dead man’s pedal is a promise made in reverse: the driver keeps it pressed, and if they let go, the brakes come on. It assumes the worst thing that can happen to a driver is that they stop pushing. In 2003, near Waterfall in New South Wales, a driver had a heart attack at the controls, and the inquiry found that the weight of his leg was enough to keep the pedal down. It also found that the legs of a large share of Sydney’s drivers were heavy enough to do the same. The fix was to stop asking for pressure and start asking for answers: a vigilance system that sounds every so often and brakes unless someone presses a button in reply. A body can hold something down without anyone in it. It takes a person to answer a question.