Final answer:
In 1860, the Southern United States and parts of the far West were the regions least served by railroads, due to limited industrialization and the impact of the Civil War in the South, and a lack of extensive rail networks in Latin America.
Step-by-step explanation:
The regions in the United States that were least served by railroads in 1860 were primarily in the South and parts of the far West. At the time, the Southern states had limited industrialization and urbanization, with much of the land used for plantation crops or by poor farmers, and the infrastructure, including railroads, suffered extensive damage during the Civil War. Additionally, Latin America, compared to the United States and Europe, had a less successful attempt at industrialization and did not develop extensive national rail networks, mainly having short lines connecting the interior to the coast for exporting raw materials.
Prior to the 1870s and 1880s, the South's rail infrastructure was sparse, with track standards incompatible with those of the North, leading to isolated transport systems. It wasn't until after the reconstruction efforts in the latter half of the 19th century that the Southern railroads began to connect with the national standard, enabling trains to operate seamlessly between regions.
Historically, better-served regions by rail were the port cities and interior areas of the northern and central United States, where the growth of rail travel transformed the economy in the 1840s and 1850s by significantly improving commerce. The development of the railroad infrastructure allowed for the expansion of markets and the distribution of goods across vast distances, thereby supporting the nation's economic growth and connectivity between states.