How does transportation function as a mid-range social cause?

Transportation systems function to move people, goods, and ideas. Rail systems, road networks, airline systems, and water transport provide links between places that permit more reliable and low-cost movement of people and goods from point to point than previously available. The history of transportation is simultaneously a history of technology change, population movement, colonialism, economic growth, business development, and the spread of disease.

Transportation systems are particularly interesting when we consider their capacity for conveying social causation. Consider these examples of causal relations mediated by transportation systems:

• Extension of a rail network stimulates the growth of new towns, villages and cities in North America in the 1880s.
• Establishment of a direct air travel link between A and B causes the more rapid spread of disease between these locations.
• Breakdown of the administration of the rail system leads to logistics bottlenecks and military defeat of the French army in the Franco-Prussian War.
• Regular river travel throughout the Canton Delta in China leads to the rapid spread of revolutionary ideas during the Republican Revolution, as travelers and merchants move easily from place to place.
• Commodity price correlation increases between Chicago and New York as a result of regular and cheap rail transport between these two cities.
• New business institutions (grain futures markets and grain elevators) are created to take advantage of cheap regular rail transport (Nature's Metropolis: Chicago and the Great West).
  

Examples can be multiplied. But the central point is that transportation is a robust causal mechanism that mediates many important social processes and outcomes. And its causal effectiveness is fairly transparent: new transportation opportunities create new options for social actors, who take advantage of these opportunities in choosing a place to live and work, in pursuing political goals, in moving armies, and in generating income. So transportation is a causal mechanism whose microfoundations are especially visible, and whose causal consequences are often very large.

(See "Transportation as a Large Causal Factor" for more on this subject.)