The signalman at Longforgan signal box stated that he put the mechanical starting signal correctly back to Danger behind the Dundee train. Around ten minutes later, the Aberdeen train arrived at his box and drew up to the Home signal, which was then cleared for it. The train continued to draw down to the Starting signal but, after a few moments, began to accelerate. It passed the Starting signal which, as far as the signalman could see, was still at Danger. He went down onto the track and saw that the arm of the Starting signal was slightly raised; about 4°. Subsequent investigations showed that it was possible for the arm to have been raised roughly 8°.