English Wikipedia has an article on:

smokebox

Wikipedia

From smoke +‎ box.

• (UK) IPA^(key): /ˈsməʊkbɒks/

smokebox (plural smokeboxes)

1. A chamber in which smoke passes from a boiler before being vented through a chimney.
   • 1944 November and December, “Modified G.W.R. "Hall" Class Locomotives”, in Railway Magazine, page 350:

     […] 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.