War and the immediate post war years. The G class worked alongside the six KB class locomotives, and were able to move tonnage which would have required fourteen [[NZR J class|J class or JA class locomotives. Only 9 trains in each direction could be run, each way, through the steep section from Arthur's Pass to Springfield, six regular freight, an extra run as required freight in both directions, the West Coast express three times a week and the overnight perishables mixed train 140/220. The express and perishables were hauled by AB class locomotives, and the remaining freight trains usually by KB class and one or two G class, or sometimes up to three G class. While difficult engines disliked by engine crews, the G class moved huge tonnage in these hard vital years, running more mileage, at lower operating costs than the A or AA classes and in ton miles were outperformed only by the J, JA, AB and KA. After the introduction in 1939 of the new KB class both the G class and the KB class were trialed on the South Island Limited and mail express on the Main South Line, during the Second World War as far south as Timaru and sometimes to Oamaru and Dunedin, once certain bridges had been strengthened to accept the 14-ton axle loading of the rebuilt G and KB class locomotives. Tests proved the G and KB with their high axle loads, unique complexities as booster or three-cylinder systems more efficiently deployed moving heavy coal trains on the Midland line. The G class simply lacked the speed to keep time. While the G class were partly redesigned with express work in mind, they were incapable of more than 50mph, let alone the consistent 60-70mph required to make up time on fast expresses over the Canterbury plains. The G class were often employed on regional stopping passenger trains, such as Christchurch - Ashburton and Christchurch- Springfield, being alright as long as the load was only three to four carriages and a couple of vans and no fast running to make up time was required, with their big (for narrow gauge) 4ft 9 inch drivers, high boiler and Pennsylvania like styling. 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 steam leaks were of particular concern