In a major train derailment, the train will often come off the tracks completely. Depending on where the derailment occurs, its momentum may cause it to collide into compartments ahead of the compartment that became derailed. If any passengers are onboard, this can result in serious injuries.

A train derailment occurs when one or more wheels of a rail vehicle leave the track. This process usually begins with a specific trigger, such as a broken rail, excessive speed on a curve, or a mechanical failure like a defective axle. During a "minor" derailment, the train might simply vibrate intensely as the wheels run along the sleepers (ties), often coming to a safe stop with little damage. However, in a "major" derailment, the loss of guidance causes the railcars to jackknife or tip over due to kinetic energy. Modern trains in 2026 are equipped with positive train control (PTC) and sensors that detect unusual vibrations, automatically applying emergency brakes to minimize the impact. Emergency responders prioritize securing the area, especially if the train is carrying hazardous materials. Passengers are instructed to remain seated until the train has completely stopped, as the secondary "whiplash" effect of cars colliding can cause more injuries than the initial derailment itself.