Bags of peanuts are one of the only food items sold on train carriages on trolleys that can cause serious reactions, given their tendency to leave crumbs and for traces to become airborne easily. This is important because many people in the UK suffer from life-threatening food allergies to peanuts.
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By not offering peanuts on trains, the environment can be made much safer for those travelling with severe and life-threatening allergies, as people will not be tempted to buy peanuts simply because they are on offer. As a peanut allergy sufferer myself, I would love to be able to commute between university and home - and work in the future - and not have this become an unnecessarily stressful experience just because of the sale of one item of food.