There are four sub-classes, all with West Midlands Railway for use on the Snow Hill Lines and Leamington Spa - Nuneaton: The 172/0s and 172/1s resemble the existing Turbostar trains in not having end gangways.

They also differ from earlier Turbostars in having mechanical transmission rather than hydraulic - gear changes can be distinctly heard as the trains accelerate and decelerate.

[15] The success of London Overground's operation of the Gospel Oak to Barking Line meant that the two-car Class 172s eventually proved insufficient in terms of capacity.

[18] They were temporarily replaced by Class 378s redeployed from other parts of the London Overground network and moved to West Midlands Trains in March 2019.

Thus, they were not able to operate on the Aylesbury via Amersham line whilst the current signalling system was in place unless running in multiple with other units on both the front and rear ends, which were fitted with a tripcock.

[21] In late 2007 and early 2008, orders were placed with Bombardier by two rolling-stock leasing companies to purchase a number of Class 172 Turbostar trains.

Work began at the end of 2009 after the freeing-up of one of Bombardier's production lines following completion of the Class 377 order for First Capital Connect.

[26][full citation needed] However, it transpired that the exhaust emission testing had been flawed and that there were no major problems with the units or the original design.

[27] The Government announced in December 2008 that Bombardier, with its Turbostar design, was one of the pre-qualified bidders for the first 200 DMU vehicles of its planned 1300 new carriages.

However, with the announcement of the electrification schemes in the North West and on the Great Western Main Line, the DMU order was cancelled, with the needs of the train operating companies planned to be met by transfers of existing stock.

Arriva Rail North and TransPennine Express (First Group) did subsequently obtain new DMU and bi-mode rolling stock, although this was produced by CAF and Hitachi and also in higher quantities - 58 CAF DMUs were allocated to Arriva Rail North alone, whereas the 61 cancelled 172s (42x 3-car and 19x 4-car) would have been split up into allocations for three operators.

[29] One unit has received a name: 172342: Roger Sumner - in honour of a driver at Worcester who retired in late 2022 after almost 60 years of service.