A Labour government would invest more than £25 billion in creating a second rail link between Brighton and London.
A new Brighton Main Line has been included in the Labour election manifesto which has been unveiled by leader Jeremy Corbyn today.
A £100,000 government study into the rail link released in March said there was "no case" to take the project forward at the current time.
The rail link had not been included in the leaked version of the manifesto from last week.
Also among the manifesto pledges, the party proposes raising an extra £6.4 billion by raising the income tax rate to 45p for people earning over £80,000 and a new 50p rate for those on more than £123,000.
The party will also bring private rail companies back into public ownership
as their franchises expire and replace the "dysfunctional water system" with a network of regional publicly-owned water companies.