Final answer:
Transcontinental rail service in the United States commenced in the 1860s, with the completion of the first line on May 10th, 1869, marked by the golden spike ceremony at Promontory, Utah.
Step-by-step explanation:
The transcontinental rail service began in the United States during the 1860s. Specifically, it was on May 10th, 1869 when the first transcontinental railroad was completed, connecting the nation coast to coast. This monumental event was celebrated with the driving of a golden spike at Promontory, Utah. Leading up to this, the First Transcontinental Railroad was financed by an 1864 grant of federal lands given to the Union Pacific and Central Pacific railroads, which built tracks between Omaha, Nebraska, and Sacramento, California, with Ogden, Utah serving as the connecting point.
By 1887, there were four major transcontinental lines linking various parts of the country. This rapid expansion was a result of the economic and industrial boom, which led to significant growth in related industries like iron, wood, and coal. Inventions such as car couplers, air brakes, and Pullman passenger cars further facilitated an increase in the volume of freight and passengers, tripling the amount of transported goods and the number of travelers on the rails from 1877 to 1890.