Bouygues, the French contractor, has bought the French government's 21% stake in train maker Alstom for *2bn (£1.4bn).
Martin Bouygues, the firm's chairman, said the deal represented an expansion into new and growing business areas.
The two companies are now expected to work together on large infrastructure projects in the transport and energy sectors.
Bouygues has agreed that it will not sell the stake for at least three years, and has said it does not intend to build its shareholding to 34% or more, which, according to takeover rules, would force it to make a bid for the whole company.
In a separate deal in France, support services group Amec has bought heating, ventilation and air conditioning firm Societe Nouvelle de Chauffage et de Climatisation. It has a turnover of about *10.5m (£7.7m), and will be incorporated within Amec Spie, which Amec has put up for sale.