There is a growing demand for public mass transportation between the Centre and the Western Zone of Rio de Janeiro that saturates the roadways. The Estate Government declares not having the means of investment to finance the best solution proposed: Rio s 4th Underground Line. The goals of this paper are to verify the problem presented and to check the viability of financing part of the project through trading Carbon Credits. Those credits would come from the economy made in this course by a large number of automotive vehicles that would refrain from launching CO2 into the atmosphere. The Underground, being electric powered, is considered to be a clean way of transportation due to the fact that in Rio de Janeiro electricity is generated in hydroelectric plants and at the two nuclear plants: Angra I and Angra II, thus this project would qualify to receive financing though the Carbon Market.