NZR Bb class

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NZR B^B class
In service	30
Road numbers	55, 109, 143, 144, 147, 167, 169, 171, 197, 222, 618-637
Builder	A & G Price
Built	1915 (12), 1916 (8), 1917 (8), 1918 (2)
Wheel arrangement	4-8-0
Weight	69 tons
Height
Width
Length	52 ft 7 1/2 in
Maximum speed	40 mph
Boiler pressure	175 lbf/in²
Tractive effort	20,940 lbf
Coal capacity	4.0 tons
Water capacity	1,700 gallons

The B^B class of steam locomotives comprised 30 engines that operated on New Zealand's national rail network. Similar in design and appearance to the preceding B and B^A classes, the first B^B class locomotive entered service in February 1915, with the last to commence operations doing so on 8 March 1917. All were built by A & G Price Ltd of Thames, New Zealand, and as their cylinders had a larger diameter than the B and B^A locomotives they were capable of generating more power to haul heavier trains. The B^B class could haul up to 700 tons of freight on a level railway line, though they were limited to a top speed of around 40 mph.

The B^B class did not solely haul freight trains. They were also utilised to haul passenger trains, generally on branch lines where light track meant trains could not be operated at speeds unattainable for the B^B class. These trains included services for miners working in coal mines along branches in the Waikato region.

In the later days of steam, powerful locomotives such as the K class were hauling heavy trains that the C class and other shunting locomotives at yards and depots simply could not handle. Accordingly, ten members of the B^B class were modified to perform shunting duties and they successfully took on the heaviest of roles.

Most B^B locomotives survived into the 1960s. During that decade, the complete withdrawal of the class was undertaken progressively, with the last two, B^B 626 and B^B 633, formally removed from service in August 1968. Another one of the last to be withdrawn was B^B 144 in October 1967, and it survived to be preserved by Mainline Steam, though it is presently inoperable. No other B^B locomotive has been preserved.

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See also: Motive power explanation; Multiple units and Railcars descriptions.