An investigation has begun after a passenger train derailed in East Sussex, leaving two people seriously injured and another 18 requiring hospital treatment. Three carriages of the Southern Railway service from London Victoria to Eastbourne left the tracks between Lewes and Haywards Heath, prompting emergency services to declare a major incident. Passengers described terrifying scenes as the train rocked violently before carriages left the rails, with one reportedly landing on its side. The Rail Accident Investigation Branch is examining the cause, while specialist teams assess damage to the train and railway infrastructure. Local MP James MacCleary said concerns had recently been raised about the route becoming noticeably “bumpier”, possibly following prolonged extreme heat. Images have prompted speculation that heat-related track buckling may have contributed, although investigators have not yet confirmed this. The railway remains closed, causing significant disruption and replacement bus services. The incident follows repeated heatwaves that have placed growing strain on Britain’s transport infrastructure this summer.