The British Rail Class 166 Networker Turbo is a fleet of diesel-hydraulic multiple unit passenger trains, built by ABB Transportation at their Holgate Road Works in York between 1992 and 1993.

They were specified by and built for British Rail, the state-owned railway operator in Great Britain at the time.

Until 2017, they operated only on express and local services in the Thames Valley area alongside the Class 165 units.

Six cars were added to the original order in 1991 after Network SouthEast acquired some of the Cotswold Line services from Regional Railways to allow Class 158 units to be converted to Class 159s for the West of England services.

[8] In July 2017, the 166s began to transfer over to be based at St Philip's Marsh depot in Bristol, where they would then begin operation on the Severn Beach line to allow other services to be strengthened further west.

Towards the end of January 2010, First Great Western announced an £8 million refresh programme to their fleet of Class 166 Turbo DMU trains.

[13] The carpets and seats were retrimmed, interiors repainted, Passenger Information Displays replaced with a GPS-based system, and toilets upgraded.

The changes made during the 2010 refresh remain in place, with some upgrades, including the fitting of LED head/tail lights, new toilets which are more accessible than the old toilets, new door buzzers & new door buttons of the standard design with a round button on a yellow ring.

Unit 166204 was named in honour of Twyford station master Norman Topsom MBE who retired in November 2015.