The Dudding Hill Line (or 'Dudding Hill Loop') is a railway line in north-west London running from Acton to Cricklewood. The line has no scheduled passenger service, no stations, no electrification, and a 30 mph speed limit with semaphore signalling, and is lightly used by freight and very occasional passenger charter trains. It is roughly 4 miles long.
The southernmost point of the Dudding Hill Line is in Acton, where it branches north from the North London Line between Acton Central and Willesden Junction stations. From there it heads north then east, passing through Gladstone Park and terminating at a triangular junction with the Midland Main Line between Cricklewood and Hendon. There are intermediate junctions with the West Coast Main Line from the south at Harlesden and the Chiltern Main Line from the south at Neasden.
More information, including route maps and proposals , on the Wikipedia page [1].