 On the night of the accident the weight of snow on some signal arms and wires meant that the arm balanced well short of the 'danger' position and that the spectacle plate failed to drop sufficiently to move a red lens in front of the white lamp displayed in the "all clear" position.The signal therefore showed a white light ("all clear") when the lever in the signal box was in fact set to danger; the snow and ice (in the words of the Inquiry Report) thus rendered the signals 
not only useless for warning by red-lights the engine-driver of the Scotch-express-train, but also a means by the exhibition of white lights of luring him forward at full speed to the collision.The enquiry had heard evidence from experienced working-level witnesses that this (or a milder manifestation of this) was a known problem on snowy days but could easily be sorted if you knew how, e.g., by "jiggling the arms".