Final answer:
The Transcontinental Railroad took six years to build due to challenging terrain, labor shortages, and engineering work.
Step-by-step explanation:
The Transcontinental Railroad was built between 1863 and 1869, taking a total of six years to complete. The Central Pacific Railroad started construction from Sacramento, California in the west, while the Union Pacific Railroad began construction from Omaha, Nebraska in the east. The challenging terrain, labor shortages, and the need for extensive engineering and surveying work contributed to the length of time it took to build the railroad.
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