In the early 1990s, the Waterloo to Exeter route was suffering many delays and problems due to the increasingly unreliable diesel locos being used at the time. It was therefore decided that new trains were the best option. At the time, there was an existing order for Class 158 units already being constructed and it was decided to modify the last 22 of these as Class 159s, specifically for use on western England services. The first of these units started work on the line during May 1993 and gradually replaced all the remaining locos and stock. Later, it was becoming obvious that the initial 22 units were no longer sufficient for the increasing number of travellers, so a further eight Class 158s made redundant in the north of England were modified as Class 159/1s and entered service in the mid-2000s. While their service continues to be good, as they are approaching 30 years old, the time is approaching for replacement.