Final answer:
On May 10th, 1869, the first transcontinental railroad was sealed with a golden spike at Promontory, Utah, marking the completion of the railroad connection between the eastern and western coasts of the United States.
Step-by-step explanation:
On May 10th, 1869, the first transcontinental railroad was sealed with a golden spike at Promontory, Utah. This event, known as the Golden Spike ceremony, marked the completion of the railroad connection between the eastern and western coasts of the United States. The railroad system grew significantly after this milestone, with approximately 180,000 miles of railroads across the nation by 1890.