The £14.8 billion cross-London Crossrail scheme could be extended, Transport Secretary Patrick McLoughlin has announced.
The extension would take high-speed train Crossrail services to Hertfordshire, he said.
If the plan goes ahead, there will be Crossrail links, providing faster commuter services, to the Hertfordshire stations of Tring, Hemel Hempstead, Berkhampstead and Watford Junction as well as to Harrow & Wealdstone in north west London.
The existing Crossrail, due to be fully operational by 2019, will see fast trains running from as far west as Reading in Berkshire through central London to as far east as Shenfield in Essex.
Mr McLoughlin made his extension announcement as he addressed Crossrail workers at Farringdon station in London.
He said: "We are looking at proposals to extend Crossrail services even further by opening a new route to key destinations in Hertfordshire.
"The plan would provide flexibility and reliability while we build HS2 into (London's) Euston (station). It would make it easier to get on with the massive job of rebuilding the station so it matches the amazing standard of (London's)King's Cross and St Pancras - because everyone who uses Euston now knows it isn't up to scratch.
"I t would be a huge boost to Crossrail and London commuters. We're going to examine, right away, how we can run extra services direct into the city and through to Canary Wharf from key stations like Tring, Hemel Hempstead, Harrow and Watford - without the frustrating need to change at Euston."
Mr McLoughlin added: " Preliminary analysis has indicated that the link could potentially reduce journey times to central London by up to 15 minutes.
"I want to get on with this work, so I have asked HS2 Ltd to work closely with the Crossrail sponsors to explore the proposals in more detail.
"I have made clear to my department that any changes must not affect the timetable or planned service pattern for the existing Crossrail scheme, which is scheduled to be fully operational by 2019.
"Of course - as with any feasibility study - final decisions would be subject to a wide range of considerations, including affordability."
Harrow Council leader David Perry said: "New investment from the Government in London's infrastructure is welcome, along with the potential regeneration opportunities this will bring to Harrow.
"We will follow developments closely and work with all organisations to ensure any extension to Crossrail supports our priorities to support local businesses and get young people into work in Harrow."
London mayor Boris Johnson said: "I have always maintained that the renaissance of Euston is a fantastic opportunity for regeneration in our city and one we should not miss.
"Providing a new rail route into the capital for passengers from Hertfordshire will be vital for Euston to be properly redeveloped to accommodate HS2 and to maximise the delivery of new homes and jobs in London and beyond."