Depending on the temperature and duration of a fire, high-level radioactive waste shipping containers could be breached. The design criteria for such containers is for a 1,475 degree Fahrenheit fire, burning for only 30 minutes. Many real world fires burn hotter, and longer, than that. A breach of a container, and the release of even a small fraction of the volatile contents of a cask (such as Cesium-137), would cause a disastrous or even catastrophic release of hazardous radioactivity into the environment.
And if a train bridge collapses above water, the underwater submersion of a high-level radioactive waste shipping container could cause a breach, depending on the depth of the water, and the duration of the submersion. As a Public Citizen backgrounder documents, design criteria for shipping containers only account for a 3 foot submersion, for a damaged container; the design criteria for undamaged casks is for a depth of 656 feet, for one hour. But many bodies of surface water are deeper than 3 feet. And the containers weighing as much as 180 tons, or more, these days, it would require a special crane to lift them -- something that would take much longer than an hour to bring in, set up, and activate. A breach and release could cause catastrophic hazardous radioactive contamination of drinkinging water supplies, fisheries, etc.
In addition, infiltrating water could spark an inadvertent nuclear chain reaction, if a critical mass of fissile material (U-235, Pu-239) forms inside the irradiated nuclear fuel container. This would potentially make emergency response a suicide mission, given the fatal doses of gamma and neutron radiation emitted by a chain reaction. But doing nothing could mean catastrophic releases of hazardous radioactivity into the surface water. See this backgrounder, for more information about such risks.