From videos I have seen of the shredder at Newport, an entire and intact railway carriage could not be put in it. The bodywork is pulled apart using a mechanical grab so that it's broken down into more manageable pieces. Looking at pictures in the HST Mk3 carriage disposals thread, it would appear that the bogies are removed before scrapping begins. I don't know about the seats but given the high volume of stock arriving at Newport, I presume these get mixed up with the piles of scrap and are sifted out, along with the non-recyclables when the bit are put through the shredder.