The line will cost an estimated €1bn and forms part of a larger project intended to link the two countries with Burkina Faso and Ivory Coast.
France's Bolloré Group, the strategic partner for the project, is mobilising funds to ensure the railway is completed within four years.
Up to 40% of the construction costs will come from strategic partners, 10% each from the governments of Niger and Benin, and 40% from private sector partners in the two countries.
The Niamey – Cotonou railway will form part of a 2500km West African rail loop which will eventually connect Benin and Niger with the cities of Abidjan in Ivory Coast and Ouagadougou in Burkina Faso.
The project is intended to boost regional economic development by facilitating exports of agricultural products, livestock and mineral resources.