From the longer Wikipedia article 
The Gospel Oak – Barking Line is a railway line in north and east London which connects Gospel Oak in North London and Barking in East London as part of the London Overground network - and also carries through freight trains. It is sometimes known as the Goblin (for Gospel Oak and Barking LINe), although this is a nickname rather than an official title. Unlike most railways in London, the line does not pass through the central Travelcard Zone 1. Traditionally the line has played a minor role in London's transport system; however, it is now receiving significant investment to increase its capacity. The line is part of the Network Rail Strategic Route 6, SRS 06.03 and 06.05. It is classified as a London and South East Commuter line.