The NZR G class was a type of Garratt locomotives used in New Zealand, later rebuilt as Pacific type locomotives. They were the only Garratt type steam locomotives ever used by the New Zealand Railways (NZR). They were ordered to deal with traffic growth over the heavy gradients of the North Island Main Trunk (NIMT) and to do away with the use of banking engines on steep grades. They were one of the few Garratt designs to employ six cylinders. A mechanical stoker was used to feed coal into the locomotive. The locomotives lasted longer in rebuilt form as standard Pacific locomotives than they did as Garratts, but their numerous mechanical issues lead to their final withdrawal following a union ban on their use in 1956.
Introduction
About 1913, the General Manager E. H. Hiley considered the importing of ten articulated Garratt engines and ten Pacifics. With the success of the AB class and WAB class Pacifics no more was heard of Garratts. Then with the retirement in 1925 of the Chief Mechanical Engineer E E Gillon his successor G S Lynde invited Beyer, Peacock & Company of England to suggest a suitable Garrett for the NIMT, and they were then asked to quote for engines with either four or six cylinders. But the three six-cylinder engines were supplied "against their own better judgement. The influence of the London & North Eastern Railway three-cylinder enthusiasts (i.e. Lynde) was evident in this unwise decision." These engines had three cylinders () on each of the two set of engine frames, thus creating a 6-cylinder Garratt. The engines entered service in 1929.
Walschaerts valve gear operated the outside cylinders with the inner third cylinder operated by a Gresley-Holcroft mechanism. The locomotives proved a disaster on the light NZR tracks. It has been suggested the most likely reason was that the engines were too powerful for the system and also the valve gear mechanisms were complicated. The design was most unusual in that the coal bunker was carried on an extension to the boiler frame rather than the normal Garratt positioning on the rear engine's frame. Unlike a Union Garratt the rear water tank was still mounted on the rear engine unit.
The engines operated at and delivered of tractive effort which, on the lightly laid New Zealand tracks, proved to be too powerful for the drawbars on rolling stock and broken drawbars occurred wherever the engines ran. Further, the locomotives when hauling a full load, generated such intense heat in restricted tunnels, which are common in New Zealand, that crews disliked working them. Their large size driving wheels also made them unsuitable for the NIMT.
The G class were mostly based at Ohakune and operated between Taihape and Ohakune on the NIMT. The central section of the NIMT of 93 miles (153 km) from Taumarunui to Taihape had been relaid with heavier 70 lb/yd (34.8 kg/m) rather than 53 lb/yd (26.3 kg/m) rails in 1901 for the introduction of the heavier NZR X class locomotives.
Withdrawal
Trainloads were reduced and this defeated the purpose for which the Garratts were purchased – namely to operate heavy loads over a vital mainline section of the NIMT route, the central section including the Raurimu Spiral. The trailing engine axle under the cab carried a heavier load than the leading engine trailing axle and experienced continual problems with overheating. Also, the coal bunker carried insufficient fuel in-service and this problem was never remedied because it would have increased the axle loads beyond the light track capabilities.
One engine (G 99) was withdrawn from service in 1935, with G 98 and G 100 following in early 1936. Their numerous design faults sealed the fate of these locomotives when the K class was introduced in 1932.
Rebuilding as Pacifics
Due to the troubles faced with the Garratts in their original form, a proposal was put forward in late 1935 for the three Garratts to be dismantled and the engine units used to build six new 4-6-2 tender locomotives. The three locomotives were dismantled at Hutt Workshops in 1936 and the engine units shipped to Hillside Workshops in Dunedin for eventual rebuilding. The engines as rebuilt were fitted with a new third cylinder, a modified AB class boiler, a new cab and trailing truck based on those used on the Baldwin AA class, and a new Vanderbilt tender based on those used on the AB class, but of welded construction and fitted with roller bearing bogies. The original plate frames were retained as was the Gresley conjugated valve gear.