Dudding Hill Line

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.

More information on the Wikipedia page.