Welwyn control prevents the block being turned to "Line Clear" while a train is in the section. A train is proved to have passed right through the section by it occupying then clearing the berth track circuit at the home signal. Having done so, "Line Clear" can be given again to admit the next train into the section. An emergency time release is provided to override the Welwyn Control in the event that "Line Clear" has been given for a train which is subsequently cancelled and does not require to enter the section. When operating this device, an enforced time delay passes before the release becomes effective, giving the signalman a period of time in which to consider his or her actions. The introduction of Welwyn Control was a recommendation of the report into a fatal rear-end collision on 15 June 1935 at Welwyn Garden City on the LNER, when the signalman had overlooked a train already in the section.
This example was manufactured by A.R.Parsons and is in ex signal box condition, these "block winders" are very difficult to source so dont let this you slip away!