Before entering traffic, the original 22 units were modified at Rosyth Dockyard to Class 159 to operate services from London Waterloo to Salisbury and Exeter St Davids, replacing various locomotive-hauled passenger trains.

[1] In the late 1980s, the locomotive-hauled stock on Network SouthEast's West of England route from London Waterloo to Salisbury, Yeovil Junction and Exeter St Davids was in urgent need of replacement.

[10] With the UK economy in decline in the early 1990s, it was found that Regional Railways had over-ordered Class 158s at the same time as Network SouthEast was looking for a similar number of new diesel trains.

The rebuild was required because it was not possible for Network SouthEast and the newly privatised BREL to agree terms on the variation order to NSE specification.

Until the December 2009 timetable change,[18] some trains continued beyond Exeter to Paignton, Plymouth and Penzance; these usually operated as three-coach units, though at weekends there were some six-coach formations.

CCTV and PIS (Passenger Information Systems) were installed,[20] new seating was installed in first class and at the same time the units received a modified version of the SWT express livery (with orange doors as opposed to the red doors on Class 444 units) for compliance with disabled access regulations.

[12] The refurbishment included making the first-class accommodation area larger and completely refitting it, brighter interior lighting with new diffusers and the plating-over of the disused toilet in the MSO vehicle.

[21] The converted units however retained their original Cummins NTA855-R1 engines, which produce 37 kW (50 hp) less power than the R3 variants fitted to the Class 159/0 fleet.