Very scientific
62 The failures of the multiple track screws were not immediately obvious when
RAIB attended the derailment. The upper part of each broken screw was held in
its associated baseplate by a plastic ferrule that had been compressed around
it during installation. These ferrules held the broken screw heads tightly in the
baseplates, so that they could not be turned by hand, and felt solid when given a
kick. The failed state would similarly have been hidden from visual inspection, and
from any test kicks from a patroller’s boots, making detection by a BVI difficult.