没人知道火车为什么会脱轨。No-one knows why the train derailed.火车脱轨栽进了河里。The train derailed and plunged into the river.火车头在拐弯处脱轨.The engine of the train left the rails at a curve....

“脱轨”的英语可以翻译为：derail，digression ...