The British Rail Class 165 Networker Turbo is a fleet of suburban diesel-hydraulic multiple unit passenger trains (DMUs), originally specified by and built for the British Rail Thames and Chiltern Division of Network SouthEast.

They are regeared and fitted with bogie yaw dampers to allow a top speed of 90 mph (145 km/h), more suitable for mainline use.

[8] The 165/0 units were originally delivered in Network SouthEast livery for used on routes including fast services from London Marylebone to Princes Risborough and Banbury and local services from Aylesbury to London and Princes Risborough.

A new depot was built at Aylesbury in 1990/1991 for the maintenance of these trains and has been enlarged since British Rail days, with the addition of a wheel lathe.

[6] Air conditioning was added and the opening hopper windows replaced with sealed units.

A new passenger information system, similar to that on the Class 168 Clubman trains, CCTV cameras and an area designated for the use of wheelchair users were added and the first-class section was removed, as Chiltern became a standard-class-only railway in 2003.

A further refurbishment began in 2015, concentrating on the toilet areas, to make these units fully Disability Discrimination Act 1995 (DDA)-compliant for operation beyond 2020.

The Class 165/1 fleet were built for local services from London Paddington along the Great Western Main Line; their main destinations included local trains to Reading, Greenford, Newbury, Bedwyn, Oxford, and Banbury, and services along the branch lines to Windsor & Eton Central, Henley-on-Thames, Marlow and Bicester Town.

In April 2004, operation of the Thames Trains franchise passed to First Great Western Link.

[14] In late 2015, as part of the rebranding to GWR, the Class 165 fleet had all first-class sections removed to increase capacity.

[7] Following the electrification of the Great Western Main Line up to Didcot Parkway, as well as the Reading-Taunton line as far as Newbury, services between London Paddington and Didcot Parkway, as well as between Reading and Newbury, have been operated by new Class 387 electric multiple units,[15] allowing much of the existing Class 165 fleet to move to the Bristol area.

Class 165s continue to service the aforementioned branch lines, but no longer run to London Paddington except during peak hours.