A subway train has smashed into the rear of another at the height of the evening rush hour in Washington DC, killing at least six people and injuring scores of others.Cars of both trains were ripped open in what has been described as the worst accident in the system’s 33-year history. Some 200 firefighters were called to the scene. Rescue workers used ladders to help survivors scramble to safety. More than 70 people have been treated for injuries and the search is continuing for more passengers trapped in the wreckage. The crash happened on the city’s red line, the busiest, which runs at ground level at the accident site in a north-eastern suburb.