Transport for London (TfL) has also revealed the number of trains that it expects to run on the line once it opens.
The West London Orbital would see passenger services introduced on existing freight rail lines that run from West Hampstead and Hendon towards Hounslow.
A recent TfL report sets out plans for six Overground trains an hour between Hendon and Hounslow. It does not reveal how many services are planned from West Hampstead.
Four of these trains would travel the full length of the line, with a further two services between Hendon and a new station called Old Oak Common Lane.
If built, the new station would allow passengers to interchange with HS2 services at Old Oak Common.
Mildmay line services could also call at the new Overground station, TfL says.
Using 2023 prices, the current cost of the scheme, which could open in 2034, is £800 million.
Funding discussions with London boroughs and the Government are ongoing, with a consultation on the plans expected at the end of the year.
Hounslow Council recently approved £714,000 worth of investment in the project.
Last November, London Mayor Sadiq Khan said: “We are going to be lobbying the Government in advance of the spring spending review.
“The point we are making is that actually these new lines will enable more homes, more jobs and growth.”
The final route is yet to be revealed, but current plans would see trains call at 15 stations in total.
Existing platforms will be used in some instances, but new stations would be required at Neasden, Harlesden, Old Oak Common and Lionel Road.
Design work is expected to be completed by next summer, which would pave the way for an application to start work on the new line in 2028.
The other stations where trains could stop are Cricklewood, Brent Cross West, Acton Central, South Acton, Kew Bridge, Brentford, Syon Lane and Isleworth.
TfL and Network Rail are working with West London Alliance to help deliver the new Overground route.
