Alex Salmond has said Labour would have to agree to extend HS2 to Scotland if Ed Miliband wants SNP backing for a minority Labour government.
The former Scottish first minister suggested Labour could be forced to make the concession in the first Budget if it wants SNP backing.
Speaking yesterday on the BBC’s Andrew Marr show, Salmond set out a list of demands it would want Labour to agree to which included work on HS2 starting north of the border.
“So I propose an amendment to [that] budget,” Salmond said.
“Let’s say instead of this very, very slow train coming up from London, I think we should start it from Edinburgh/Glasgow to Newcastle and I put that down as a budget amendment. It would have substantial support from the north of England and other parties and would carry the House of Commons. What does Mr Balls do then?”
He added the Scottish National Party could work with the Greens and Welsh party Plaid Cyrmu to force through “progressive” spending commitments. This could mean a substantial SNP group of 40 to 50 MPs in Westminster.
Labour and SNP leaders have both ruled out forming a coalition but Salmond has predicted working with them on a “vote by vote” arrangement.
Leaders of Labour and the SNP have both ruled out the possibility of the parties working together in Westminster in a formal coalition, but have not rejected the possibility of collaborating as part of a looser arrangement. SNP leader Nicola Sturgeon has spoken of working with others to “lock out the Tories”.
Scottish Labour leader Jim Murphy told the BBC his party was "still confident" of winning the election outright.
And he predicted the SNP would feel the "full fury of the Scottish people if they saved David Cameron's skin".
The current HS2 project, which has come under fire from both political sides, would link London to Birmingham by 2026, with tracks to cities in the north of England including Leeds and Manchester, built by 2030.
Train journey times between two of the north of England’s biggest cities could be cut to 26 minutes if the so-called HS3 line is built.
Last week, chancellor George Osborne unveiled multi-billion pound plans to improve key rail and road transport networks in the north of England.
The new Trans North long-term rail project aims to reduce train travel times by building high-speed track on routes between key northern cities such as Liverpool, Manchester, Leeds, Sheffield, Newcastle and Hull.