The 57km (35-mile) twin-bore Gotthard base tunnel will provide a high-speed rail link under the Swiss Alps between northern and southern Europe.
Switzerland says it will revolutionise European freight transport.
Goods currently carried on the route by a million lorries a year will go by train instead.
The tunnel has overtaken Japan's 53.9km Seikan rail tunnel as the longest in the world and pushed the 50.5km Channel Tunnel linking the UK and France into third place.