RA 8777 BR WR PARSONS WELWYN BLOCK RELEASE WINDER INSTRUMENT

On 15 June 1935, a train from London King`s Cross to Leeds collided with the rear of a train from Kings Cross to Newcastle at night. Fourteen people were killed and 81 injured, 29 seriously. The accident was a caused by a Signalman`s  error: the signalman at Welwyn Garden City, who had been fairly recently appointed to the box, became confused and accepted a second train into a block section that was already occupied. The Newcastle train, arriving first, received a signal check and was slowed to 15–20 mph; the Leeds train, consisting of 11 coaches hauled by Class K3 2-6-0 No 4009, ran into it at approximately 65 mph.

 the Inspector recommended that the block instruments should be linked to the track circuits  to prevent future occurrences in such a way that a "Line clear" indication could only be given on the block instrument if the track circuits had registered passage of a train; this was widely adopted and known as Welwyn Control. This instrument is known as a Welwyn Block Release, its purpose was to be able to reset the block indicator from Line Clear if line clear was given and needed to be cancelled. An example of this would be a failed train, The signalman at the box in advance could only reset the block once this instrument had been wound for approximately 2 minutes or until the "N" appeared in the small aperture.  This example was manufactured by Parsons and used on the former G.W.R. lines.