Difference between B7-4 and B7-6: The B7-6 has standard rubber gangways on one end, to be coupled to the FR7 (bistro) which has the same rubber gangways so that it can be coupled to sleeping-cars in the night. That is because some express trains are split up in the evening, and a few cars including bistro become part of the night train. The B4 has airtight gangways which are not compatible to the rubber gangways.