A The primary purpose of the smokebox on any steam locomotive is to enable the cylinder exhaust to exit the locomotive. At the same time the hot vapors from the firebox are drawn through the boiler tubes to both give draft for the fire and to enable the tubes to heat the water in the boiler.
The dimensions of a smokebox on any given locomotive are designed to create the most efficient drafting of the exhaust and the hot vapors from the firebox. Coal-burning engines often had longer smokeboxes than oil-burning engines due to railroads using various types of coal. Some soft types of coal did not burn as hot as harder coal and thus there was a need to increase the drafting of the smokebox to create a hotter fire in the firebox.
Many times, crews would find that a certain locomotive was not a good steaming engine because it did not appear to draft well. In those occasions it was not unusual for the railroad’s shop workers to redesign the smokebox. On other occasions, if an engine was superheated, the shop may need to add length to the smokebox to give room for the superheater header and pipes.
The railroads that were most likely to modify smokeboxes on their locomotives were those that bought engines secondhand from other lines. Such railroads may vary in operating terrain from which the secondhand engines were originally designed to handle and may, especially in the case of logging railroads, change the type of fuel used in a locomotive.
At the turn of the 19th century, it was not unusual for mainline railroads to modify smokeboxes to increase their drafting abilities. This often coincided with changes in the fuels used. Such railroads tended to have their locomotive designs set by the 1920s and few roads altered the smokeboxes on new engines they bought after that time. – Martin E. Hansen
