Explanation:
The draught in locomotive boilers is produced by steam jet.

Draught:
Draught is the flow of air and gases through the combustion of fuel. Draught is necessary for the proper combustion of fuel in the boilers. It is the pressure difference between the inside and outside of the boiler that causes the gases to flow.

Types of Draught:
There are two types of draught: natural draught and artificial draught.

Natural Draught:
Natural draught is the draught produced by the chimney. The hot gases produced by the combustion of fuel are lighter than the cold air outside and, therefore, rise up the chimney. The rising gases create a low-pressure area inside the boiler, which causes the air to flow in through the grate.

Artificial Draught:
Artificial draught is the draught produced by mechanical means. It is used when the natural draught is not sufficient to create the required draught in the boiler. There are two types of artificial draught: steam jet draught and mechanical draught.

Steam Jet Draught:
Steam jet draught is produced by injecting steam into the chimney. The high velocity of steam creates a low-pressure area inside the chimney, which causes the air to flow in through the grate. The steam used for this purpose is obtained from the boiler itself.

Mechanical Draught:
Mechanical draught is produced by using fans or blowers. The fans or blowers are used to force the air into the boiler. This type of draught is used in large boilers, where the natural draught is not sufficient to create the required draught.

Conclusion:
The draught in locomotive boilers is produced by steam jet. The high velocity of steam creates a low-pressure area inside the chimney, which causes the air to flow in through the grate.