The Class 170 diesel multiple unit (DMU) is a development of the design used in the British Rail Class 165 and 166 DMUs known as the Networker Turbos and built by British Rail Engineering Limited and later ABB Transportation before that company became part of Bombardier.

They are unable to operate in multiple with units in the 16X series due to different wiring arrangements.

Class 170 units have been categorised into six (originally seven) sub-classes; the basic specifications remain the same (engine, length, transmission, etc.

[12] In April 2015, the ScotRail franchise passed from First to Abellio, and nine units owned by Eversholt went off-lease.

[14] Arriva Rail North began operating the Class 170 in 2018, with a total of 16 three-car units (170453-170461 and 170472-170478) received from Abellio ScotRail by January 2019.

These can now be seen operating services between Sheffield and Scarborough via Hull and have diagrams around the Harrogate Loop, on which they share duties with Class 158 DMUs.

East Midlands Railway operates the largest fleet of 170s, currently consisting of 43 total two-car and three-car sets.

From 23 March 2020, East Midlands Railway received its first two Class 170 units in the form of three-car 170416 and 170417, when they moved from Abellio ScotRail.

[15] In September 2022, three Class 171s, having been reformed to three-car formations and renumbered to 170422–170424, transferred from Southern to East Midlands Railway.

[18] They regained their identities as 170/4s when BSI were fitted in May 2023 and they can now work with the rest of the fleet instead of being on self contained diagrams.

[19] CrossCountry currently operates 29 Class 170 Turbostars (seven two-car and 22 three-car) on services between Cardiff Central and Nottingham, and between Birmingham, Leicester, and Stansted Airport.

The extra centre cars were re-liveried and internally refurbished to match the units they were inserted into.

From late 2006 to 2016, First TransPennine Express operated nine Class 170s, used on the Manchester Piccadilly to Hull route.

Originally Class 185s were set to operate the route but a combination of weight-restriction problems on the Selby to Hull Line, chronic overcrowding on several of the company's services and the government reducing the amount of money available to First TransPennine Express for new trains resulted in Class 170s coming into service.

In 2005, following its acquisition by FirstGroup, Hull Trains received four new Class 222/1 Pioneer units and transferred the Turbostars to its sister company First ScotRail.

Midland Mainline (MML) was the first operator to order Turbostars, the first being delivered in November 1998 and entering service in May 1999.

MML ordered a fleet of 17 two-car Class 170 units, although the first ten were subsequently made up of three cars each instead.

The units were introduced on stopping services from London St Pancras to Leicester, Derby and Nottingham.

In 2004, Midland Mainline introduced new Class 222 Meridian units, which started to replace the Turbostars.

After the Central Trains franchise ended, the two three-car units (170397 and 170398) remained with the new operator, CrossCountry.

When Central Trains lost its franchise in 2007, 23 of Central Trains' units (17 two-coach and six three-coach) were transferred to London Midland, which took over the West Midlands franchise and continued to use Class 170s for services on the Chase Line, Birmingham to Hereford via Bromsgrove Line and Shrewsbury services.

The first batch of eight three-car Class 170/2 units were built between 1999 and 2000 for London Liverpool Street to Ipswich, Norwich, Lowestoft and Bury St. Edmunds services.

Other units, including the spot-hire set 170399, were used on Anglia's short-lived Norwich to Basingstoke 'London Crosslink' service.

Units operated on London Waterloo to Salisbury, Reading to Brighton, Southampton local services, and occasionally on services to Exeter St Davids, though this was not a regular route for these units, as they do not feature end gangways, making it difficult to provide trolley services, and they lacked selective door opening for the short platforms at stations on the route west of Salisbury.

Unit 170392 was transferred back to its original intended operator Southern and has since been reclassified and renumbered from 170392 to 171730.

The first five units then remained on hire to Abellio ScotRail via a sublease arrangement until March 2020, while 170421 to 170424 moved to Wolverton Works in April 2015.

They will be converted back to Class 170s and resume their original identities, while the fourth will remain on sublease to Govia Thameslink Railway.

[31] The centre coach of WMT's six Class 170/6s moved to CrossCountry in 2021 to enable it to strengthen some of its two-car sets.

These were originally due to stay with TfW for the duration of the franchise (albeit working different services once their brand new trains were in service), however following a revision of TfW's long-term rolling stock strategy, it was later announced all were to transfer to East Midlands Railway.