Harriett has campaigned with her Westminster colleagues to invest in the important train line to London which is still single track over a stretch of line between Charlbury and Oxford.
And the PM has written to Chancellor George Osborne and Transport Secretary Patrick McLoughlin asking them to commit funds to the project before the end of this Parliament in 2015.
Harriett commented: “The complete dualling of the Cotswold Line to Oxford is an ambition of mine and together with my colleagues Sir Peter Luff and Robin Walker, we have consistently called for this important investment in our transport infrastructure.
“This is an important intervention by the Prime Minister, who does use the service and is aware of the problems rail commuters to London face every day.
“The dualling of the track would allow the service providers to run faster, more frequent and more reliable services which would be a massive boost to the local economy.
“I think that these letters to the two people who can green light the project is great piece of news and I hope that it will lead to even better news in the future.”