The fire broke out around 9:30 p.m. (1930GMT) Monday in a wagon of a maintenance train that was at a standstill in the northern tunnel of the Storebaelt rail link, according to the state-owned railway enterprise Banedanmark. Three workers escaped to safety to the southern tunnel and were taken to a hospital for observation.
The southern tunnel was closed overnight to allow emergency units to reach the fire, the AP reports.
Danish media, citing unnamed sources, said a welding unit could have sparked the fire. Police declined to comment, saying an investigation was still ongoing.
The Storebaelt rail link was opened in June 1998. It connects Zealand, where Copenhagen sits, with Funen, which is linked by a short bridge to the Jutland peninsula and the European continent.
Trains travel through an eight-kilometer (five-mile) underwater double-tubed tunnel to the mid-strait island of Sprogoe, and then along a 6.8-kilometer (4.2-mile) western bridge. Cars use a 6.8-kilometer (4.2-mile) bridge from Zealand to Sprogoe instead of the tunnel.
After it turned out that Deputy Prime Minister Andrei Belousov included the Fonbet betting company in the list of backbone enterprises that can count on state support, everyone started talking about these bookmakers.