British Rail Class 166

British Rail Class 166 Networker Turbo Express
Great Western Railway unit 166208 at Didcot Parkway
The refurbished interior of Standard Class
In service	1992 - Present
Manufacturer	ABB York
Family name	Networker
Constructed	1992 - 1993
Number built	21 trainsets
Formation	3 cars per trainset
Fleet numbers	166201 - 166221
Operator(s)	Great Western Railway
Specifications
Car body construction	Welded aluminium
Maximum speed	90 mph (145 km/h)
Prime mover(s)	One per car, Perkins 2006-TWH Diesel
Power output	350 hp (261 kW)
Transmission	Voith Hydraulic T211r 2 axles driven per car
Safety system(s)	AWS, TPWS
Coupling system	BSI
Track gauge	1,435 mm (4 ft 8 1⁄2 in) standard gauge

The British Rail Class 166 Turbo Express is a fleet of diesel multiple units (DMUs), originally specified by and built for British Rail, the then United Kingdom state owned railway operator. They were built by ABB at York Works between 1992 and 1993. The trains were designed as a faster, air conditioned variant of the Class 165 Turbo, intended for longer distance services, and, like the 165s, belong to the Networker family of trains. They were originally known as Networker Turbos to distinguish them from the electrically propelled members of that family.

The class is still in service, and is extensively operated by Great Western Railway on its services out of London Paddington station. The trains, along with that operator's Class 165 trains, are often known as Thames Turbos.

These units are a modification of the Class 165 design. They have a top speed of 90 mph (145 km/h) (suitable for mainline use), are carpeted throughout and have air-conditioning. Externally, the class 166 can be distinguished from a Class 165 by opening hoppers on every other window. Until late 2013 the presence of first class at each end was another distinguishing feature.

Other differences over a 165 are as follows:

Twenty-one 3-car units were built, numbered 166201-221. Each unit was formed of two outer driving motors, and an intermediate motor. The technical description of the formation is DMCO+MSO+DMSL. Individual carriages are numbered as follows:

The units were built to replace elderly Class 117, Class 119 and Class 121 DMUs, and locomotive-hauled trains on services from London Paddington along the Great Western Main Line.