6 , ▁returned ▁to ▁work ▁on ▁the ▁main ▁line , ▁operating ▁both ▁service ▁trains ▁and ▁char ter ▁trains . ▁D 9 0 0 0 ▁' Ro yal ▁Sc ots ▁Grey ', ▁the ▁do yen ▁of ▁the ▁class , ▁was ▁returned ▁to ▁main ▁line ▁service able ▁status ▁in ▁ 1 9 9 6 ▁and ▁continued ▁to ▁work ▁throughout ▁the ▁UK ▁for ▁nearly ▁twenty ▁years . ▁Following ▁a ▁power ▁unit ▁failure ▁this ▁locomot ive ▁was ▁fitted , ▁for ▁a ▁time , ▁with ▁an ▁ex - N or weg ian ▁Navy ▁T 1 8 - 3 7 K ▁type , ▁after ▁various ▁modifications ▁were ▁clever ly ▁designed ▁to ▁make ▁the ▁new ▁unit ▁compatible . ▁ ▁Fire ▁department ▁use ▁The ▁New ▁York ▁City ▁Fire ▁Department ▁used ▁a ▁Nap ier ▁D elt ic ▁engine ▁to ▁power ▁their ▁one - of - a - kind ▁" Super ▁P um per ▁System ". ▁This ▁was ▁a ▁very - high - volume ▁tra iler - mount ed ▁fire ▁p ump ▁with ▁a ▁separate ▁tender . ▁ ▁R eli ability ▁in ▁service ▁ ▁While ▁the ▁D elt ic ▁engine ▁was ▁successful ▁in ▁marine ▁and ▁rail ▁use ▁and ▁very ▁powerful ▁for ▁its ▁size ▁and ▁weight , ▁it ▁was ▁a ▁highly ▁str ung ▁unit , ▁requiring ▁careful ▁maintenance . ▁This ▁led ▁to ▁a ▁policy ▁of ▁unit ▁replacement ▁rather ▁than ▁repair ▁in ▁situ . ▁D elt ic ▁engines ▁were ▁easily ▁removed ▁after ▁break down , ▁generally ▁being ▁sent ▁back ▁to ▁the ▁manufact urer ▁for ▁repair , ▁although ▁after ▁initial ▁contract s ▁exp ired ▁both ▁the