The normal way engineering is done is to create a safety margin so that the probability of a serious problem under normal operation is vanishingly small. Airlin

The normal way engineering is done is to create a safety margin so that the probability of a serious problem under normal operation is vanishingly small. Airliners do not fall out of the sky on a regular basis in the United States for that reason. Serious problems have become rare through serious application of serious standards. Engineering rail systems so that derailments do not happen at all is a reasonable shorthand for almost never. It is engineering malpractice to run things so close to the edge that fatalities or serious property damage are to be expected next month instead of sometime in the next few decades or so.