arp , ▁Stewart ▁and ▁Co . ▁Ess entially ▁they ▁were ▁develop ments ▁of ▁the ▁firm ' s ▁previous ▁batch ▁of ▁ 4 - 6 - 0 s ' ▁for ▁N Z R . ▁ ▁Origin ▁and ▁design ▁ ▁Due ▁to ▁an ▁increase ▁in ▁traffic ▁around ▁the ▁turn ▁of ▁the ▁century , ▁N Z R ▁obtained ▁ 4 7 ▁ 4 - 6 - 0 ▁locomot ives ▁to ▁provide ▁additional ▁capacity . ▁All ▁were ▁used ▁in ▁the ▁South ▁Island . ▁The ▁U C ▁class ▁were ▁the ▁last ▁new ▁ 4 - 6 - 0 ' s ▁to ▁be ▁delivered , ▁entering ▁service ▁between ▁August ▁and ▁October ▁ 1 9 0 1 ▁. ▁They ▁were ▁briefly ▁class ified ▁U ▁. ▁Extern ally ▁they ▁were ▁similar ▁in ▁appearance ▁to ▁Sh arp ▁Stewart ' s ▁earlier ▁U A ▁class , ▁but ▁with ▁a ▁longer ▁cyl inder ▁stroke ▁and ▁consider ably ▁higher ▁bo iler ▁pressure ▁( 2 0 0 ▁p si ▁as ▁opposed ▁to ▁ 1 7 5 ▁p si ) ▁they ▁had ▁a ▁significantly ▁higher ▁tract ive ▁effort . ▁All ▁of ▁the ▁locomot ives ▁began ▁service ▁in ▁the ▁Christ ch urch ▁area ▁aside ▁from ▁two ▁allocated ▁to ▁the ▁Dun ed in ▁- ▁In ver c arg ill ▁section . ▁ ▁Service ▁ ▁With ▁the ▁opening ▁of ▁the ▁O ti ra ▁T unnel ▁in ▁ 1 9 2 3 ▁all ▁ten ▁were ▁gradually ▁transferred ▁to ▁the ▁West ▁Coast ▁of ▁the ▁South ▁Island . ▁Due ▁to ▁weight ▁restrictions ▁they ▁were ▁the ▁most ▁powerful ▁main line ▁locomot ives ▁on ▁the