NZR BB class

NZR BB class
Power type	Steam
Builder	A & G Price, Thames, NZ
Serial number	63–92
Build date	1915 (12), 1916 (8), 1917 (8), 1918 (2)
Configuration	4-8-0
UIC classification	2'Dh
Driver diameter	42.5 in (1.080 m)
Length	52 ft 7 1⁄2 in (16.04 m)
Weight on drivers	32.5 long tons (33.0 t)
Locomotive weight	43.5 long tons (44.2 t)
Tender weight	25.5 long tons (25.9 t)
Fuel type	Coal
Fuel capacity	4 long tons (4.1 t)
Water capacity	1,700 imp gal (7,700 l; 2,000 US gal)
Boiler pressure	175 psi (1.21 MPa)
Firegrate area	16.8 sq ft (1.56 m2)
Heating surface: Total	724 sq ft (67.3 m2)
Superheater area	208 sq ft (19.3 m2)
Cylinders	Two, outside
Cylinder size	17 × 22 in (432 × 559 mm)
Top speed	40 mph (64 km/h)
Tractive effort	20,940 lbf (93.1 kN)
Number in class	30
Number	55, 109, 143, 144, 147, 167, 169, 171, 197, 222, 618–637
Preserved	1 (BB 144)

The B^B class of steam locomotives comprised 30 engines operated by New Zealand Railways in the North Island of New Zealand. 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 Another one of the last to be withdrawn was B^B 144 in October 1967, and it survived to be preserved by Mainline Steam, though it is presently inoperable.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.

External links

• New Zealand Railways Steam Locomotives - Class B^B
• Official website of Mainline Steam, owners of B^B 144