Prior to the withdrawal of their full length 2+8 HST sets in May 2019, GWR experimented with 2+4 formations. Two variations emerged: the slam door sets, and the sliding door sets. The slam door sets became known informally as the class 257s, whilst the sliding door sets became known (somewhat more formally) as class 255s.

The slam door sets were withdrawn along with the full length sets in 2019, however the sliding door sets remain in service, over 5 years since the 2+8s were withdrawn.

Initially, GWR retained a fleet of 36 power cars to run these sets. However, these have been withdrawn over time and now only 12 remain in service, with 3 scrapped, several exported, and the rest stored.

Below is a table of all 36 power cars as well as the current status of each power car.