Final answer:
Early American railroads transformed the transportation infrastructure and facilitated a national market system. They were not concentrated in the Southeast and cooperated with each other while also having differences in schedules, depots, and equipment.
Step-by-step explanation:
The early American railroads had a significant impact on the country's economy in the mid-19th century. They transformed the transportation infrastructure, linking port cities to the interior and facilitating the emergence of a national market system. Contrary to option a, the railroads were not concentrated in the Southeast but expanded across the country. Option b is partially true; while individual railroad companies competed with each other, they also cooperated to establish common standards like track gauges and schedules. Option c is incorrect as rail travel was not only affordable to the richest passengers, but it also provided a more accessible mode of transportation for a wider range of people. The correct answer is option d; early American railroads had different schedules, depots, and equipment as the system was still being developed.