The oldest variant of the Networker family that was built in series was the class 165. To distinguish it from the electric variants, it was later called “Networker Turbo”. They were ordered by Network SouthEast for service in the Thames Valley and Chiltern Region and are therefore also known as “Thames Turbos” and “Chiltern Turbos”.

The sets consist of two or three cars, all of which are powered. Under the floor of each car is a six-cylinder turbo diesel from Perkins with a displacement of 12.2 liters and an output of 350 hp. The power is transmitted to both axles of a bogie via a hydraulic gearbox from Voith. The top speed is basically 75 mph. 

The basic variant 165/0 was built 28 times as two-car sets and eleven times as three-car sets. Some of the trains use part of the London Underground lines and have therefore got the tripcock system. In addition, the class 165/1 was built 18 times as two-car sets and 19 times as three-car sets. These received a modified gear ratio for 90 mph and yaw dampers on the bogies to be used on main lines.

For longer distances, the class 166 was built from 1992, which, like the 165/1, can reach 90 mph. In addition to an adapted interior, this has air conditioning and additional toilets in contrast to the 165. Here 21 sets were created, all of which have three cars.

Since the privatization of British Railways, the trains have changed operators several times. Today all are owned by the Great Western Railway and Chiltern Railway. Later, many Class 165 trains also received air conditioning and interior modernization, while the first class compartments were partially replaced with second class. Some trains were moved to the Bristol area following the electrification of the Great Western Main Line, others were replaced with class 168 or class 180 diesel multiple units and then moved to more remote areas. To date, only one class 165 set has been scrapped after an accident.