Final answer:
The tragedy that befell Mr. Mallard is that he died in a train accident.
Step-by-step explanation:
Based on the given information, it is believed that the tragedy that befell Mr. Mallard is that he died in a train accident. In the story, it is mentioned that Mrs. Mallard's husband's name was listed as one of the 'killed' in a railroad disaster. This suggests that Mr. Mallard lost his life in a train accident.