Final answer:
The Main Line is associated with the historical expansion of the railroad network in the 19th century, particularly in relation to cities like Harrisburg and Philadelphia, symbolizing the broader economic and urban growth supported by advances in transportation.
Step-by-step explanation:
The Main Line referenced in the question refers to the key railroad routes established during the growth of the United States' transportation network in the 19th century. As the Transcontinental Railroad network was developed, major cities like Harrisburg played crucial roles in the expansion of rail lines both eastward and westward. This development was pivotal in connecting different regions of the country, facilitating westward migration, and supporting economic growth through improved trade and transportation.
Understanding the historical context of these railroad lines, we can see the profound impact they had on urban development. Cities like Philadelphia - near Harrisburg - flourished in part because of their connections to this expanding railroad network. These railroads facilitated the growth of 'walking cities' into larger metropolitan areas as the rise of streetcar suburbs expanded the urban footprint. Thus, the Main Line is reflective of the broader pattern of economic and urban development driven by advancements in transportation technology during this era.