Final answer:
Railroads were the form of transportation that led to national horse racing competitions. Developed in the 1830s and gaining prominence in the 1840s and 1850s, they reduced travel times and connected various parts of the country, facilitating the movement of horses and enthusiasts to racing events across the nation.
Step-by-step explanation:
The form of transportation that led to national horse racing competition was railroads. The development of railroad infrastructure beginning in the 1830s fundamentally transformed American society, including the landscape of horse racing. As rail travel became more prominent in the 1840s and 1850s, it linked port cities to the interior, thereby creating a national market system that included the racing industry. Before railroads, travel and communication were limited to slower methods, such as horseback or horse-drawn vehicles. With the advent of the transportation revolution, which saw roads, turnpikes, canals, and eventually railroads rapidly constructed, the time required to travel greatly decreased. This enabled horses and enthusiasts to travel to different parts of the country for racing events, leading to national competitions. This growth was further enhanced by the introduction of the steam locomotive, exemplified by the first locomotive used to pull cars in the United States, the Tom Thumb.