Final answer:
Steel was preferred for railroad tracks and locomotive boilers due to its strength, durability, and the decreased costs associated with its production following advancements such as the Bessemer and open-hearth processes.
Step-by-step explanation:
Steel was the preferred material for both railroad tracks and locomotive boilers. This preference is due to steel's increased strength and durability, especially after the Civil War when the Bessemer process and open-hearth process made the production of large quantities of inexpensive steel possible. The capability of steel to support heavier locomotives pulling heavier loads was essential to the expansion of railroads across the United States and Europe, thereby transforming the transportation infrastructure and fueling the growth of commerce.
The availability and advantages of steel led to its use in manufacturing rail lines that were heavier and stronger, enabling them to handle the demands of a rapidly growing railroad industry. The innovations in railway construction contributed to more efficient and powerful locomotives able to navigate the unique terrain and challenges of the American landscape, such as steeper grades and tighter curves, thanks to the durable and resilient steel infrastructure.