The British Rail Class 166 Networker Turbo is a fleet of diesel multiple-unit passenger trains (DMUs), originally specified by and built for British Rail, the then Great Britain 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. Today the 166s, alongside the 165s, are normally referred to as Thames Turbos or just simply Turbos. The Class 166s are still in service today, solely operated by Great Western Railway. Until 2017, they operated only on express and local services in the Thames Valley area alongside the Class 165 units. In this time, they were based at Reading TMD but since July 2017, the 166 units have been gradually moved over to be based at St Philip's Marsh depot to operate local and regional services around Bristol and Exeter. Nowadays, the majority of the 166s are based in Bristol while many 165s remain in the Thames Valley. Description These units are a modification of the Class 165 design. They have a top speed of (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: Air conditioning Two toilets (a 165 only has one toilet per unit) Tables in first class and in one third of the middle carriage Dedicated cycle/luggage storage in the middle carriage Different interior panelling between the door and seating areas 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 DMCL-MS-DMSL. Individual carriages are numbered as follows: 58101-58121 - DMCL 58601-58621 - MS 58122-58142 - DMSL (changed from DMCL in 2013) The units were built to replace Class 117, Class 119 and Class 121 DMUs, and locomotive-hauled trains on services from London Paddington along the Great Western Main Line. Six cars were added to the original order in 1991 after Network SouthEast acquired some of the Cotswold