For railroad bridges, what is the best type of girder?

The best type of girder for railroad bridges is a box girder primarily due to its structural efficiency and performance characteristics. Box girders are closed-section beams that provide excellent resistance to bending and torsional stresses while maintaining a lightweight structure. This is particularly advantageous in railroad applications where the girders must support not only the static loads from the bridge itself but also the dynamic loads from passing trains.

Box girders offer heightened stability and load distribution, which enhances structural integrity during the vibrations and forces exerted by train traffic. Their design allows for a significant reduction in material usage without compromising on strength. Additionally, their shape is less susceptible to buckling compared to some open-section girders, which is crucial in heavy-duty applications like those found in railway systems.

The other types of girders, while potentially useful in specific contexts, do not match the overall balance of strength, efficiency, and lightweight characteristics provided by box girders in the context of railroad bridges.