What structural feature is common to firebox boilers and locomotive boilers?

Firebox boilers and locomotive boilers share a common characteristic in their construction principles, which is why the correct answer focuses on this aspect. Both types of boilers are designed to generate steam effectively by using a combustion chamber to house the fire, allowing the heat produced to transfer efficiently to the water. This design principle maximizes the heating surface area and ensures a more effective steam generation process.

In firebox boilers, the firebox is where fuel is burned, and the resulting hot gases pass over the water to heat it. Similarly, locomotive boilers are designed with a firebox that serves a comparable purpose in generating steam for propulsion. The similarities in heating and safety principles are essential for their functionality in various applications.

Understanding the construction principles helps in recognizing how both boiler types achieve efficient heat transfer and steam generation. Other features such as vertical arrangement, standardized dimensions, or integrated heating elements may not apply universally to both boiler types, whereas the underlying construction principles remain a central aspect of their design.