Final answer:
The expansion of railroads and the establishment of supportive legislation like the Homestead Act were the primary drivers of the significant surge in western settlement during the 1860s and 1870s.
Step-by-step explanation:
The expansion of railroads contributed most significantly to a surge in western settlement during the 1860s and 1870s. The completion of the Transcontinental Railroad in 1869, coupled with the Homestead Act of 1862, which offered 160 acres of free land to households willing to move west, greatly facilitated westward migration. This period also saw an increase in available farm equipment and the establishment of forts to aid settlers, further encouraging the move to the West.