Controversial plans for a 26-billion-euro high-speed rail link between France and Italy will be at the centre of talks between leaders of the two countries on Monday.
French President Francois Hollande and Italian Prime Minister Mario Monti are determined to press ahead with a scheme that has generated furious opposition from environmental activists on both sides of the Alps.
Both leaders see the project as emblematic of the pro-growth agenda they have jointly promoted within the European Union. This shared outlook has helped the two countries come closer than they have been in decades.
Supporters of the scheme claim it will take a million heavy lorries off the saturated roads between Italy and France, as transAlpine freight switches to rail, cutting CO2 emissions by three million tonnes per year.
But the plans for a new tunnel under western Europe's highest mountains have also come under fire.
Critics argue it could become a white elephant subsidised by unjustifiable injections of national and European funds at a time when every other area of public spending is being tightened.
It is now 21 years since the idea of the link was launched at a previous Franco-Italian summit.
Back then, there was much talk of the economic benefits that would be generated by halving journey times between Lyon and Turin to two hours; and cutting the Paris-Milan time from seven to four hours.
But while the first trains are currently scheduled to travel on the new line in 2028 at the earliest, French officials acknowledge that the target may slip as budget constraints bite.
The existing Mont-Cenis rail tunnel between the two countries was built in the 19th century. It is regarded as inefficient because it was built at an altitude of 1,300 metres -- and on a slope. That means up to three train engines are required to get heavy cargoes through.
The new 57-km (35-mile) tunnel, at 500m altitude, will have easier access.
But while it was initially supported by green groups, they have since turned against it. Hundreds of activists are expected in Lyon to make their opinions heard.
"We should start by making sure the existing facilities are being used at full capacity," said Pierre Meriaux, a Green regional councillor on the French side of the border.
The Mont-Cenis tunnel currently operates at only 30-percent of its capacity.
In Italy, the No-TAV (Treno Alta Velocita) movement has been much bigger than the protests in France. It has broadened its focus to combatting the austerity programme imposed by Monti's government in response to the Eurozone debt crisis.
"The No-TAV movement is a source of inspiration for every activist in the country," said Giulietto Chiesa, one of the movement's leaders.
And the campaigners take heart from a report by France's public spending watchdog, the Cour de Comptes.
The report, published in early November, savaged the poor management, vague financing arrangements and rising costs associated with the project. The government, it said, "should not hastily rule out the alternative of upgrading the existing line."
That statement has increased the pressure on Hollande and Monti to reaffirm their commitment to the project: but they could struggle to convince sceptics that the financing is in place.
Of the total projected cost, around one third (8.5 billion euros) is assigned to the tunnel: as a cross-border project it is eligible for EU funding.
Paris and Rome are pushing want that contribution to get the maximum, 40-percent possible. But that has been thrown into doubt because of the uncertainty over the 2014-20 EU budget, which is still to be agreed. Britain and Germany are currently blocking any increase above inflation.