It really happened. In Switzerland, no less. Spiegel Online explains how a Swine Flu Container Explodes on Train:

According to the police, a lab technician with the Swiss National Center for Influenza in Geneva had travelled to Zurich to collect eight ampoules, five of which were filled with the H1N1 swine flu virus. The samples were to be used to develop a test for swine flu infections.

The containers were hermetically sealed and cooled with dry ice. However, it seems the dry ice was not packed correctly and it melted during the journey. The gas coming from the containers then built up too much pressure and the ampoules exploded, as the train was pulling into a station.

It was not the mutated form of the virus, fortunately. I wonder how often ampoules explode. An American Scientific Glassblowers Society Safety and Hazards Committee report suggests these explosions are a known risk.

Throughout our careers as scientific glassblowers, we have witnessed several incidents involving cryogens that have caused injury to personnel. By sharing these experiences, it is our hope that we will all have a greater understanding and respect for cryogenic liquids.

The obvious solution in this case would be a pressure alarm requirement for containers, especially those with dry ice that are known to have explosive risks.