Now part of the London Overground system, largely following the route of the North London Railway. It originally extended from Richmond Station to North Woolwich Station, but the route was cut back to Stratford Station in conjunction with the Docklands Light Railway extension, and with a branch from Gospel Oak Station to Barking Station. It was the only rail link to be shown on the Underground Map, and was unusual that it was the only line that did not enter Travelcard Zone 1
The Wikipedia article is at [1], and on the North London Line City Branch [2].