It requires that Switzerland has invented Machine Tools and owns the province of Lucerne (ID 606). Switzerland owns the province in the beginning of the game. It gives 4 levels of railroad in Lucerne as well as 10 prestige. Illogically the decision doesn't actually require you to have researched any level of railroads, and the decision can actually give a higher level of railroad, than would be possible to build with the current technology.
The Gotthard Rail Tunnel was built between 1871 and 1881, connecting the towns of Göschenen and Airolo. The first tunnel ever to be built under the Gotthard Massif, the Gotthard Tunnel was in its time considered an architectural achievement made possible by modern technology.