A bulge, knob, swelling, spine, or anything that protrudes.
For the most part they were small standard gauge 0-6-0 side tanks of the type illustrated, with long tapered chimneys and an unusual feature for the Continent in the shape of domeless boilers, the protuberance just behind the chimney being a sandbox.
From French protubérance, from Latin prōtubērantia (“bulge; protuberance”), from prō + tūber (“swelling; protuberance”) + -antia (“-ance”).
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