Would have thought the buffet must count as catering - similar to paying VAT in a restaurant but not in Tesco. But there is more confusion when the food is not hot and whether it is for consumption 'on the premises'. I would imagine all the train counts as the premises on the grounds that you are unlikely to buy your sandwiches at the buffet and then wait till you get home to eat them! But what happens when they sell you an apple which is hardly catering and conceivably you might take home, just don't know!
Food bought at the stand up buffet bar falls under the same rules as from a takeaway shop, and the rest of the train is not "premises". Hot food will be standard rated (as it is heated to order), so will most cold drinks, confectionary and crisps, but cold sandwiches will be zero rated.