Abraham lincoln’s primary impact on the construction of the transcontinental railroad was to

sign the Pacific Railroad Act into law.

Explanation:

In 1862, then-US President Abraham Lincoln signed the Pacific Railroad Act into law. This was this president's biggest milestone in building the transcontinental railroad that greatly improved US trade and transportation, allowing product transportation to take place in a shorter time and ensuring that products arrived at their destination with good quality. It also allowed producers' transportation expenses to be reduced, ensuring better priced products, which increased sales and boosted the country's economy.