What was Eads' most notable engineering challenge?

Eads' most notable engineering challenge was the construction of the St. Louis Bridge, officially known as the Eads Bridge, which spans the Mississippi River. This was a significant feat of engineering due to several factors, including the size of the river, the complex engineering required for the bridge's design, and the innovative techniques Eads employed, such as the use of steel instead of iron and the introduction of the arch design for greater stability. The Eads Bridge was one of the first major bridges to utilize these materials and techniques, making it a landmark in the history of civil engineering. Its completion was pivotal not only for transportation but also for demonstrating the potential of new engineering methods, influencing future bridge designs.

The other options, while they involve interesting engineering challenges in their own right, do not capture the magnitude and historical significance of Eads' work on the St. Louis Bridge. The construction of underground railways, the design of floating bridges, and the creation of earthquake-resistant structures are all relevant engineering topics, but none match the groundbreaking nature and impact of the Eads Bridge on both engineering practices and urban infrastructure at the time.