The deal relating to the financing of the high-speed trans-alpine link between Lyon in France and Turin in Italy was a further sign of the pair's commitment to the 26-billion-euro scheme that has generated furious opposition from environmental activists.
Police cordoned off several streets around the building in Lyon where the two leaders were meeting, as protesters gathered in the French city for a march due to begin around 1 am.
Hollande and Monti see the planned 57-kilometre tunnel under the Alps as emblematic of the pro-growth agenda they have jointly promoted within the European Union.
Supporters of the scheme claim it will take a million heavy trucks off the saturated roads between Italy and France, as trans-alpine freight switches to rail, cutting carbon dioxide 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 subsidized by unjustifiable injections of national and European funds at a time when every other area of public spending is being tightened.
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.