Early train routes connected which regions?

The correct choice highlights the critical role early train routes played in connecting key regions of the western United States. The trains established vital links between Texas and New Mexico, facilitating not only the movement of people but also trade and resource transportation. This connection was crucial for integrating the economies of these areas and encouraged migration and settlement.

Additionally, the routes extending into Arizona and California represented a broader network that supported the development of the southwestern economy as part of the expanding rail system. This network was instrumental in the westward expansion, as it allowed for the easier movement of goods and people, thus contributing significantly to the growth and development of these regions during a pivotal period in American history.

Other choices refer to routes that either do not accurately reflect historical connections or are too broad. For instance, while there were certainly train connections in the Rocky Mountain states, the specificity of the connection between Texas, New Mexico, and California is essential in understanding the early influences of rail transport. Similarly, there wasn't a direct connection established early on from New Mexico to Canada that significantly influenced settlement or commerce in the same way as the highlighted regions.