The main factor that slowed construction of the central pacific railroad was

a shortage of supplies, which had to be shipped around South America

Explanation:

Complete Question:The main factor that slowed construction of the central pacific railroad was

a shortage of supplies, which had to be shipped around South America.

intense summer heat that melted the railroad ties and tracks.

scarcity of wood used for building railroad ties and trestles.

conflicts with American Indians who closed down the tracks.