The British Rail Class 171 Turbostar is a type of diesel multiple unit (DMU) passenger train built by Bombardier Transportation at Derby Litchurch Lane Works in England. It is identical to the Class 170, except for the replacement of the BSI coupler with a Dellner coupler. This provision was made to allow emergency joining with Class 377 DC third-rail electric units, which Southern runs extensively on most lines. The units work on the southern regions of the British railway system, operating services from London Bridge to Uckfield and from Eastbourne to Ashford International, these routes being unelectrified between Hurst Green Junction and Uckfield and between Ore and Ashford International, respectively. The fleet is operated by Govia Thameslink Railway on Southern services. Design The Southern fleet is fitted with coupling devices (Dellner) different from other Turbostars, and is classified as Class 171 for this reason. The four-car units were built with the new couplers, but Class 170 two-car units were fitted at a later date, and reclassified as Class 171/7 after the modification. Three more two-car units were later acquired, but these were built from new with the new couplers. Southern gained 170392 from South West Trains. The coupler on this unit has also been changed to the Dellner type. The unit has been reclassified and renumbered from 170392 to 171730. This unit was first delivered carrying Southern livery and the number 170727 (because SWT needed another 170 and it was cheaper to place another unit on the Southern order than place a new order). It never operated in this form and was transferred to South West Trains where it became 170392. These trains have electronic onboard displays and automated next station announcements. Seating Class 171s have mainly standard-class seating plus either nine first-class seats on a two-car or 18 on a four-car; all first-class seats are reclining and have AC power sockets adjacent to the seats. Standard-class seating includes five full-size folding seats in the area by the universal toilet on the DMOCL(W) vehicle. With these seats folded there is room for wheelchair users and bicycles. Each passenger door vestibule area also has two flip-down seats. Operations Southern acquired an initial fleet of six two-car Class 171/7 and six four-car Class 171/8 units, to replace the slam-door Class 205 and Class 207 "Thumper" fleet. The first units were introduced in 2003 on the Marshlink Line from Ashford to and were