This paper examines changes in travel behaviour associated with ridership on the Jubilee Line Extension in east London and the resulting impacts on greenhouse gas (GHG) emissions. The paper looks at initial changes in mode choice after the line opened in 1999 and on-going mode share trends through to 2011. The initial mode shift is assessed through an analysis of published travel survey data and the annual TfL Rolling Origin Destination Survey. Longitudinal changes in mode share are assessed using the London Travel Demand Survey and the relationship between metro accessibility and mode choice. From 2000 to 2011 the calculated GHG savings are 338 ktCO2e; approximately equivalent to the annual average GHG emissions of 43,000 UK residents.