Hendrie was tasked to produce a locomotive of greater power and capable of longer distances without refuelling or rewatering to work the mainline's steep 1 in 30 (31⁄3%) gradients.

The NGR's faith in Hendrie's ability turned out to be well justified, however, considering the fact that some of these locomotives remained in service for over seventy years.

[1][7] Delivered in 1904, the Class B 4-8-0 Mastodon type was the first tender locomotive to be placed in service by the NGR apart from the single home-built 4-6-2TT engine Havelock of 1888.

To prevent the fire from entering the lower row of tubes, Hendrie arranged a vertical firewall towards the front of the grate, which also created a dry combustion chamber.

275 had run its trials on 25 October 1904, it was fitted with a Pyle National Electric headlight which consisted of a self-contained turbine and dynamo and an arc lamp, placed on top of the smokebox in front of the chimney and supported by two brackets.

The headlight proved so successful that it gradually replaced the old huge oil-burning lamps, which had been in use for over fifty years, on all mainline locomotives.

[1][2] Compared to the Reid Tenwheeler, the Class B was able to haul 7.5% more load and, on average, ran 26,000 miles (41,843 kilometres) more before requiring repairs.

It was fitted to all engines which were subsequently designed by Hendrie and remained the standard reverser on South African steam locomotives well into the 1940s.

[1][2][4] In their later years they were relegated to shunting, particularly working in Natal but also in Cape Town, Port Elizabeth, East London and in Transvaal.