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 (Bergische Stahl Industrie) couplers, which means that they can mechanically couple with Southern's Class 377 Electrostar EMUs for rescue purposes. Variants References Further reading External links Detailed description of the Turbostars Bombardier Transportation multiple units Adtranz multiple units British Rail diesel multiple units Train-related introductions in 1998