NZR G class (1928)
|NZR G class (1928)|
A G class locomotive in Garratt form.
|NZR G class (1937)|
The NZR G class was a type of Garratt steam locomotive used in New Zealand, the only such Garratt type steam locomotives ever used by New Zealand Government Railways. They were ordered to deal with traffic growth over the heavy gradients of the North Island Main Trunk 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.
About 1913, General Manager E H Hiley considered the importing of 10 articulated Garratt engines and 10 Pacifics; however with the success of the NZR AB class and NZR 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 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 finger of the London and North Eastern three-cylinder enthusiasts (i.e. Lynde) was evident in this unwise decision. 
In 1928, New Zealand Government Railways obtained and operated three unusual Garratt locomotives in the 4-6-2 + 2-6-4 layout from Beyer, Peacock and Company of the United Kingdom. These engines had three cylinders (16.5-by-24-inch or 419-by-610-millimetre) 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. W. W. Stewart, in his book When Steam was King (pp. 98–104) suggested the most likely reason was because the engines were too powerful for the system and also the valve gear mechanisms were complicated. Stewart stated, and existing photos verify, that 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 unit's frame. Unlike a Union Garratt, however, the rear water tank was still mounted on the rear engine unit.
The engines operated at 200 psi (1,400 kPa) and delivered 51,580 lb (23,400 kg) 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.
Train loads were reduced and this defeated the purpose for which the Garratts were purchased - namely to operate heavy loads over a vital main line 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.
In 1937 the engines were withdrawn from service. Their numerous design faults sealed these locomotives' fate 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.
The first rebuilt locomotive, G 96, was outshopped on 8 September 1937 and dispatched north after initial tests to Christchurch for use on the Midland line. Some minor adjustments were required although the performance of the initial rebuild was deemed satisfactory and the other five engine units were subsequently rebuilt with the last locomotive, G 100, outshopped on 4 March 1938. The rebuilt locomotives were initially used between Springfield and Arthur's Pass, but were later displaced in 1939 by the new KB class 4-8-4s, leading to their redeployment on the Main South Line as far south as Timaru once certain bridges had been strengthened to accept the 14-ton axle loading of the rebuilt G class engines.
Although powerful, the G class had a low adhesive factor and had issues notably with steam blows created by excessive movement of the thin plate frames. The resultant steam leaks were of particular concern to the Enginedrivers, Firemen and Cleaners' Association (EFCA), as was the lack of power-reversing gear, the latter being remediated in 1941 when Ragonnet power-reversing gear was installed. Although said to run well if kept in good repair, the G class were highly unpopular and the EFCA resolved that the class could not be used in regular service after 31 March 1956 due to visibility concerns created by the steam leaks.
Because the locomotives would need to be radically rebuilt as two-cylinder 4-6-2s, a highly costly proposal which was not seen to be worth the effort, the decision was made to retire the now badly worn-out G class locomotives after reaching a certain mileage. Both G 96 and G 97 were withdrawn in March 1956 as having reached their allotted mileage; the remaining four locomotives however remained in service until the end of May 1956 despite their deteriorating condition owing to the lack of available replacement locomotives.
Despite being withdrawn in 1956, several G class locomotives were not scrapped straight away but remained at Linwood locomotive depot in Christchurch until the early 1960s, when they were broken up for scrap.
- McClare, E.J. (1978). The NZR Garratt Story. Wellington: New Zealand Railway and Locomotive Society. p. 31. ISBN 0-908573-05-7.
- Pierre, W.A. (Bill) (1981). North Island Main Trunk: An Illustrated History. Auckland: A.H. & A.W. Reed. pp. 131, 133, 155, 156. ISBN 0-589-01316-5.
- E. J. McClare, The NZR Garratt Story, NZR&LS 1978. Pp. 86-96.