First class seats were three across with the aisle down the middle in a 1+2 configuration. Economy class seats were be 2+2. Seats were fixed and could not be rotated. This meant that some passengers would be travelling backwards.
The first batch of trailers were built with NHA bogies which are essentially the same as the BT10 bogies fitted to the HST. Some later modifications to the NHA bogies were adopted by British Rail.