smokebox

Etymology

smoke + box

noun

1. A chamber in which smoke passes from a boiler before being vented through a chimney.
   […] with this arrangement the cylinders are cast separately from the saddle, and bolted to the frames on each side with a fabricated stiffener between the frames. This stiffener is carried up to form the saddle for the smokebox, and within it is fitted the exhaust pipes from the cylinders to the blast pipe. 1944 November and December, “Modified G.W.R. "Hall" Class Locomotives”, in Railway Magazine, page 350