Final answer:
In 1870, railroads in the United States were mainly concentrated in the Central and Eastern time zones, with major developments like the First Transcontinental Railroad establishing links between the West and East Coast.
Step-by-step explanation:
In 1870, the railroads in the United States were concentrated primarily in the Central and Eastern time zones. This was the era when the First Transcontinental Railroad was completed, linking the West Coast with the existing network of railways. The Central Pacific and the Union Pacific Railroads were instrumental in this development, connecting Omaha, Nebraska, and Sacramento, California. Despite the absence of standardized time zones at this time, it was evident that railroads in the eastern half of the country were more established by 1870. Standardization of time zones was not formally adopted until 1883 when the United States was divided into four standard time zones, shaped in part by the demands of the railroad industry for more coordinated scheduling.