A smokebox is one of the major basic parts of a steam locomotive exhaust system. Smoke and hot gases pass from the firebox through tubes where they pass heat to the surrounding water in the boiler. The smoke then enters the smokebox, and is exhausted to the atmosphere through the chimney. Early locomotives had no smokebox and relied on a long chimney to provide natural draught for the fire but smokeboxes were soon included in the design for two specific reasons. Firstly and most importantly, the blast of exhaust steam from the cylinders, when directed upwards through an airtight smokebox with an appropriate design of exhaust nozzle, effectively draws hot gases through the boiler tubes and flues and, consequently, fresh combustion air into the firebox. Secondly, the smokebox provides a convenient collection point for ash and cinders ("char") drawn through the boiler tubes, which can be easily cleaned out at the end of a working day. Without a smokebox, all char must pass up the chimney or it will collect in the tubes and flues themselves, gradually blocking them.
The smokebox (outlined in red) of Soo Line 1003.
Steam Locomotive 61264 at Crown Point TMD. The smokebox can be clearly seen, with the number affixed to the smokebox door.
A smokebox with ash residues
Model steam locomotive showing spark arrester mesh
Steam locomotive exhaust system
The steam locomotive exhaust system consists of those parts of a steam locomotive which together discharge exhaust steam from the cylinders in order to increase the draught through the fire. It usually consists of the blastpipe, smokebox, and chimney, although later designs also include second and third stage nozzles.
Simple blastpipe arrangement
Later double chimney, with Kylchap blastpipes