They were introduced by Network SouthEast to replace "heritage" DMUs on the former WR lines out of Paddington and the former WR/GC routes out of Marylebone. They were built to a loading gauge wider than the BR norm, and as such betray their WR inclination. They are now also used on the Reading-Redhill-Gatwick Airport route, a firmly Southern line which (between Reading and Redhill at least) was the former province in BR days of class 206 (3R) "Tadpole" DEMU units and before that Maunsell moguls (N Class, U class & U1 class) and short rakes of BR Mk1 or SR Maunsell coaches.

The route is now operated by Thames Trains, a train operating company whose roots are distinctly non Southern! However as the line is former Southern territory we have decided to grant the classes honourary Southern status.

In terms of their welded aluminium construction and cab ends both classes are similar to the Southern's contemporary classes 465/466 and 365 EMUs. (Also the EWS class 325 used on Royal Mail services.) They are all part of the "Networker" family of units commissioned in the heady days of Network SouthEast. The Networker family was conceived as a range of EMUs and DMUs to cover all future NSE requirements but due to the prevailing economic climate and the long hiatus in rolling stock procurement caused by the privatisation process the concept never expanded beyond the classes mentioned here.

The class 165 is the inner suburban version consisting of two car (DMCL-DMS) and three car (DMCL-MS-DMS) formations. The class 166 is the air conditioned outer suburban three car (DMCL(A)-MS-DMCL(B)) version. Despite these suburban designations the units are worked as far as Worcester! This results from the Network SouthEast deal to obtain additional 158 units originally intended for Regional Railway's Paddington-Worcester services for Waterloo - Exeter services (rebuilt as class 159). Units are gangwayed within the unit only and have twin leaf swing plug doors. Car dimensions are 23.5 x 2.85m. All cars have one Perkins 2006-TWH engine providing 260kW (350hp) at 1900rpm. Drive is to one BREL P3-17 powered bogie, the other bogie being of the BREL T3-17 design. Maximum speed is 90mph. Units 165006-165039 are fitted with tripcocks for working over London Underground Limited tracks between Harrow-on-the-Hill and Amersham. Both classes can work in multiple with each other and the Chiltern Railways class 168 "Clubman" units which are an air conditioned, higher powered, slightly higher maximum speed, longer distance derivative.