Draft is the flow of combustion products going from the firebox, through the boiler and up the stack

Chimney

A chimney is a structure for venting hot flue gases or smoke from a boiler, stove, furnace or fireplace to the outside atmosphere. Chimneys are typically vertical, or as near as possible to vertical, to ensure that the gases flow smoothly, drawing air into the combustion in what is known as the...

. Draft may be natural, produced by the rising of the combustion gases in the stack, or by mechanical means, for example a blower

Fan (implement)

A hand-held fan is an implement used to induce an airflow for the purpose of cooling or refreshing oneself. Any broad, flat surface waved back-and-forth will create a small airflow and therefore can be considered a rudimentary fan...

. Since the stack of a locomotive

Steam locomotive

A steam locomotive is a railway locomotive that produces its power through a steam engine. These locomotives are fueled by burning some combustible material, usually coal, wood or oil, to produce steam in a boiler, which drives the steam engine...

 is too short to provide natural draft, during normal running forced draft is achieved by directing the exhaust steam from the cylinders through a cone (“blast pipe

Blastpipe

The blastpipe is part of the exhaust system of a steam locomotive that discharges exhaust steam from the cylinders into the smokebox beneath the chimney in order to increase the draught through the fire.- History :...

”) upwards and into a skirt at the bottom the stack. When the locomotive is stationary or in a restricted space “live” steam from the boiler is directed through an annular ring surrounding the blast pipe to produce the same effect.