Classification of a locomotive (railway) boiler Choose the correct type to complete the sentence: A locomotive boiler is a __________ boiler.

Introduction / Context:
Steam locomotive boilers powered railways for over a century. Knowing their construction helps distinguish fire-tube and water-tube categories and explains operational characteristics such as steam-raising rate, maintenance, and allowable pressure.

Given Data / Assumptions:

• Traditional locomotive boiler with firebox, tubes, smokebox, and natural draft via a blast pipe.
• Multiple small-diameter fire tubes carry hot gases; the boiler shell holds water.
• Applications involve frequent load variation and high evaporation rates.

Concept / Approach:
In a fire-tube boiler, combustion gases pass through tubes that run through a large water-filled shell. The locomotive boiler is the classic multi-tubular, internally fired fire-tube unit with a large firebox (combustion chamber) and a bank of small tubes leading to the smokebox and chimney. Superheater elements may be placed in some of the larger flues, but the core layout remains fire-tube.

Step-by-Step Solution:
Identify gas path: from firebox through tubes to smokebox—this is characteristic of fire-tube design.Check water location: water surrounds the tubes within the cylindrical shell.Confirm operational features: natural circulation, blast-induced draft, and rapid steam raising—all hallmarks of locomotive fire-tube boilers.Conclude that the locomotive boiler is a fire-tube boiler.

Verification / Alternative check:
Historical and engineering texts describe the locomotive boiler as a horizontal, multi-tubular, internally fired fire-tube boiler with a large grate area to burn coal (or oil) and produce high rates of steam for traction.

Why Other Options Are Wrong:
Water-tube boilers route water through tubes and are used for high-pressure stationary or marine service, not classic locomotives. Once-through and supercritical Benson designs are advanced water-tube variants far beyond locomotive practice. Electric resistance boilers use immersed elements and are unrelated to locomotive design.

Common Pitfalls:
Confusing the presence of a superheater (sometimes fitted) with water-tube classification; assuming “high performance” implies water-tube construction.

Final Answer:
fire tube