The immediate cause was the accumulation of snow on the semaphore arms and/or control wire of several sets of signals. 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