The TfL line (we usually call it the Orange train) runs through to New Cross Gate along the National Rail tracks, then up to and beyond Canada Water. From CW you can pick up the Jubilee Line eastbound to Canary Wharf and Stratford (great for the Olympics and Westfield Stratford) or westbound to London Bridge, Waterloo, Westminster, Bond St etc. Or you can stay on the Overgound line to Shoreditch or Highbury & Islington.

Here's a map of the Overground line.
http://www.tfl.gov.uk/assets/downloads/l...rk-map.pdf