| axleload = Route Availability 1
| displacement = per engine
History and design
thumb|left|Class 159 unit in Network SouthEast livery departing from Southampton (1996)
In the late 1980s, the locomotive-hauled stock on Network SouthEast's (NSE) West of England route from to , and was in urgent need of replacement. The diesel locomotives were not suited to the stop-start nature of the route and frequently broke down. A single breakdown could cause chaos because of the long sections of single track west of Salisbury, following the Beeching cuts.
Various options were considered, including electrification, shortened HSTs, construction of new locomotives and stock (a passenger version of the proposed Class 48), or the proposed Class 171 (which would have been part of the Networker, an inter-city version of the – not to be confused with the later Turbostars. A study found the best options were electrification or new DMUs.
The original 22 units were built as Class 158 units, but were rebuilt by Babcock Rail in Rosyth Dockyard before entering traffic. This entailed fitting first-class accommodation and retention toilets, and various other modifications. 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.
The first unit (159004) was handed over to NSE on 6 January 1993.
In 2007, eight further Class 159 units were created through the rebuilding of surplus Class 158 units displaced from the TransPennine Express franchise.
The units feature BSI couplers, which enables them to work in multiple with other units from the same class, but also and the classes of the Pacer and Sprinter groupings.
- On 31 October 2021, South Western Railway Class 159 unit 159102 collided with a Great Western Railway Class 158 train at Fisherton tunnel in Salisbury. As a result of the damage caused following the incident, two carriages of 159102 (vehicles 57803 and 58703) were written off and scrapped. Starting in 2000, units were progressively refurbished and repainted from NSE's blue, red and white livery into South West Trains' express livery. Other post-privatisation modifications included clearer LED destination displays, upgraded air-conditioning and more openable windows.
Currently, the Class 159s operate mainly from London Waterloo to Salisbury/Exeter in formations of six, eight, or nine coaches (2 × Class 159, 2 × 159 plus 1 × 158, or 3 × 159 respectively) and between Salisbury and Exeter in three- or six-coach formations. Until the December 2009 timetable change, some trains continued beyond Exeter to , and ; these usually operated as three-coach units, though at weekends there were some six-coach formations. These services are now operated by Great Western Railway. The service to is now also operated by Class 159s.
Since 2006, the original Class 159 fleet of 22 has been supplemented by eight three-coach Class 158s (renumbered into the 159/1 series) and 11 two-coach versions. The decision to standardise on 158s and 159s allowed the nine Class 170 Turbostar units to be transferred to other operators:
- London Waterloo and Salisbury to , via or Yeovil Junction.
Refurbishments and conversions
alt=|thumb|First class interior of a South Western Railway unit
Refurbishment of the Class 159/0
2000
South West Trains (SWT) began a refurbishment programme for its 22 Class 159/0s in 2000; the seats were retrimmed and interiors repainted and the units were repainted into SWT livery.
2008
The units received another refurbishment in 2008 at Wabtec Doncaster. CCTV and Passenger Information Systems (PIS) were installed, new seating was installed in first class and a modified version of their express livery (with orange doors as opposed to the red doors on units) for compliance with disabled access regulations.
Class 158 conversions
right|thumb|Standard class interior of a Class 159/1 unit in South West Trains colours
Eight of TransPennine's surplus three-coach Class 158 units were refurbished to match SWT's existing Class 159 units at Wabtec Doncaster,
The converted units retained their original Cummins NTA855-R1 engines, which produce less power than the R3 variants fitted to the Class 159/0 fleet.
Fleet details
{| class="wikitable"
!Class
!Operator
!Qty.
!Year built
!Cars per unit
!Unit nos.
!Notes
|-
!159/0
|rowspan=2|South Western Railway
|align=center|22
|align=center|1992–1993
|rowspan=3 align=center|3
|159001–159022
|Original fleet
|-
!rowspan=2|159/1
|align=center|7
|rowspan=2 align=center|2006–2007 (converted)
|159101, 159103–159108
|rowspan=2|Converted from Class 158
|-
|Scrapped
|align=center|2 vehicles
|159102
|}
Named units
Some units have received names:
- 159001 City of Exeter
- 159002 City of Salisbury
Liveries
{| class="wikitable"
! align="left" |Operating Company
! align="left" |Livery
|-
|South Western Railway
|757x757px
|-
|South West Trains
|757x757px
|-
|Network SouthEast
|757x757px
|}