Letter: Briefly

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The Independent Online
In "Driver blamed for fatal rail crash was on 'risk register'" (27 October) you mention the automatic train protection (ATP) system, which stops trains if they are driven past a red light. It is not installed on our railways because of its enormous cost - more than pounds 400m. The London Underground has used a simple and very effective system of this kind for more than 80 years. What is wrong with that?

Arthur Tarrant

Twickenham, Middlesex