Look at this image below. Do you know why and how exactly it happened?
The image above shows what happens when the rain passes through a faulty rail switch.
In this example, the railway switch deactivated before the whole carriage passed through the switch point. The switch took the front of the carriage to the right track and deactivated before the rare of the carriage passed through the switch point. Therefore, the rare of the train continued on the left track.
Such faulty switch was the reason behind Wisconsin derailment in 1996. In this accident, the train with large quantity of hazardous material derailed and caught on fire. Consequently, the city where it happened had to be fully evacuated.