The Turbostar and Electrostar platforms are a modular design, which share the same basic design, bodyshell and core structure, and are optimised for speedy manufacture and easy maintenance. They consist of a common underframe, which is created by seam-welding a number of aluminium alloy extrusions, upon which bodyside panels are mounted followed by a single piece roof, again made from extruded sections. The car ends are made from glass-reinforced plastic and steel, and are huck-bolted onto the main car bodies. Underframe components are collected in ‘rafts’, which are bolted into slots on the underframe extrusion. The mostly aluminium alloy body gives light weight to help acceleration and energy efficiency. Much of the design of the Class 168/170/171 is derived from the Networker TurboClass 165 and Class 166 trains built by British Rail Engineering Limited's Holgate Road carriage works. Notable features shared are the aluminium alloy frame and two-stage Voith T211r hydrodynamic transmission system. The diesel engine has changed to an MTU 6R 183TD13H. A cardan shaft links the output of the gearbox to ZF final drives on the inner bogie of each vehicle. The engine and transmission are situated under the body; one bogie per car is powered, the other bogie unpowered. The Class 172 differs mechanically to its older relations in several ways. The engine used is the more powerful and cleaner MTU 6H1800R83, the transmission is the mechanical 6-speed ZF Ecomat-Rail, and lighter Bombardier FLEXX-ECO bogies and hollow axles are used as well as half-height airdams. The exhaust system is also quieter and does not have the distinctive note of that of the Class 168/170/171. Turbostars have been acquired for use by several train operating companies, each with different specifications. One of the more noticeable differences with later units compared to earlier ones are the larger headlights now specified for safety reasons. Units are numbered 168 xxx, 170 xxx, 171 xxx, or 172 xxx, where xxx is the serial number of the unit. Individual carriages are numbered 50xxx and 79xxx for driving motor cars, and 54xxx, 55xxx and 56xxx for centre cars.
Working in multiple
One factor which contributes to the popularity of the Turbostars is that Class 170 units are fully capable of working in multiple with older types from the Class 15x Sprinter series of units as well as other units of the same class, unlike all other types built since privatisation, giving them greater flexibility. However, there are issues with so-called "sandwich" formations, formed either as 170-15x-170 or 15x-170-15x, which causes problems with empty stock movements where up to four units of various types coupled together is common. A possible side effect of this is that Turbostars' performance is in line with the second generation 15x units, in fact being somewhat slower than a Class 150 or Class 156 on "short hop" workings, and slower than a Class 158 on longer distance workings unless there is enough 100 mph running to take advantage of the 10 mph higher top speed of the Class 170. The Class 171 units are fitted with Dellner couplers rather than BSI couplers, which means that they can mechanically couple with Southern's Class 377 Electrostar EMUs for rescue purposes.
