Chiltern Railways - Chiltern Railways Rolling Stock - K

• Between 2003 and 2005, all 39 Class 165s were overhauled by Bombardier at Ilford EMU Depot; during this work, air conditioning units were installed throughout the fleet, while the hopper opening windows were removed; a first-class seating area was also implemented.

• During April 2003, Chiltern restored a single Class 121 Bubble Car for use on the Aylesbury to Princes Risborough shuttles. In May 2011, a second Class 121 followed after being overhauled at Tyseley TMD.
• During March 2013, a third Class 121 (121032) was acquired from Arriva Trains Wales for use as a spare parts donor. On 19 May 2017, the Chiltern Bubble cars ran for the last time, as their age made spare parts increasingly difficult to obtain.
• In January 2008, Chiltern ordered four two-coach Class 172 Turbostars; these entered service during June 2011. Because they cannot be fitted with the tripcock safety equipment necessary to operate on London Underground lines, none of the Class 172s can operate via Amersham unless attached to a Class 165 or Class 168 DMU.

• Starting in December 2010, Chiltern began operating two peak-hour loco-hauled services consisting of a Class 67 diesel locomotive hauling a rake of Mark 3 coaches and a Driving Van Trailer. A single set was initially hired from fellow DB Regio subsidiary Wrexham & Shropshire to operate a Birmingham Moor Street to London Marylebone service while another was made up of some DB-Regio-owned Mark 3s painted in British Rail blue/grey and Virgin Trains livery and operated from Banbury to London Marylebone.

• During 2012, the ex-Wrexham & Shropshire Mark 3 coaches were overhauled and fitted with sliding plug doors (similar to those fitted to South West Trains' Class 444 Desiros) and toilet retention tanks by Wabtec's Doncaster Works. Once this work was completed, a further ten Mark 3s were overhauled by Wabtec to create a fifth set and to extend the existing sets up to six coaches.
• In 2012, the DB-Regio-owned Mark 3s used on the peak-hour Banbury service were given a light overhaul at LNWR, Bristol, and the Virgin-liveried examples repainted into British Rail blue/grey. To allow Class 168s to be cascaded to the new Oxford services as part of Evergreen 3, more Mark 3s are to be returned to service in the future.
• Between 2015 and 2016, Class 170 Turbostars with First TransPennine Express transferred to Chiltern Railways. Chiltern took five of the First TransPennine Class 170s from May 2015 and the remaining four in July 2016. Upon delivery, the Class 170s were modified to allow them to be used with Chiltern's existing Class 168s, and reclassified as Class 168/3s.
• In early 2014, Chiltern Railways agreed a sub-lease from Direct Rail Services (DRS) for six Class 68 locomotives for use on its London Marylebone to Birmingham service to replace the existing fleet of Class 67s. Maintenance and commissioning for the new fleet is assisted by DRS.
• During 2017, Chiltern Railways entered into an agreement with Artemis Intelligent Power and the Rail Safety and Standards Board to provide one of its redundant DVTs as a donor vehicle for conversion into a new type of diesel locomotive.
• In July 2021, Chiltern Railways converted a single Class 168 unit (168329) into a Battery Hybrid train, referred to as "HybridFlex"; during February 2022, it was first run on services between London and Aylesbury. The company promoted the HybridFlex as increasing acceleration while reducing fuel consumption, vibration, and noise emissions, along with lessening the environmental impact of rail travel.