The new stretch of track will be built from Slough to Heathrow and would mean travellers from the south west and Wales would not have to journey to London Paddington to get to the airport.
The plan was announced in a draft consultation document on the Government’s long-term strategy for aviation in the UK.
But a decision on the controversial question of building new runways in the London area has been delayed and now is unlikely to be made until after the 2015 election.
Backers of a third runway at Heathrow welcomed that plan, and and claimed it suggested support in government for keeping the airport as the country’s main hub.
Greening said: “We are providing funding for a new rail line from the Great Western main line near Slough to Heathrow which could provide significantly improved connections from the Thames Valley, the west of England and Wales to the airport and journey time savings of up to 30 minutes.”
“The next stage will be looking to the industry on a proposed route and it should be in operation by early 2020.”
She added: “A separate call for evidence on how to maintain the UK’s international connectivity and hub status will follow later this year once the industry has had time to consider the measures in today’s consultation.”
According to previous Government statements this was due to be launched before Parliament broke up for the summer.