Final answer:
A Transcontinental Railroad would likely be built to connect colonies if they joined Confederation, drawing from the historical context of the transportation revolution in the 19th century where railroads played a crucial role in uniting different regions of the United States.
Step-by-step explanation:
If the colonies joined Confederation, what would be built to connect them would likely be a Transcontinental Railroad. During the transportation revolution prior to the Civil War, new technologies such as the steamship and railroad lines transformed the transport systems and, by extension, the economic and social connections between different areas. In the 19th century, the push for internal improvements, including roads, canals, and most pivotally, railways, aimed to forge stronger links and promote progress throughout the United States.
Railroads, in particular, played a crucial role in this. The governments provided land grants and low-interest loans to assist railroad companies in developing the nation's railway network. With the completion of the Transcontinental Railroad, which connected Omaha, Nebraska with Sacramento, California, a coast-to-coast network of rails truly united the nation. This facilitated not only the movement of goods and people but also played a part in national expansion and settlement patterns.
Given that one of the key components was the building of extensive railway systems that connected various parts of the United States, it is reasonable to infer that a transcontinental railway would be the most likely and historically consistent method to unite colonies if they were to join a Confederation.