What was the greatest challenge to completing a rail line to the West Coast?

The Central Pacific and Union Pacific Railroad's experience many unspeakable challenges along the ... The Chinese immigrant population had grown on the west coast due to the discovery of gold. ... In the end, 11 tunnels were completed. An entire camp was taken out by an avalanche, including all of its workers. Another challenge they faced was the need to create tunnels through the mountains. Using nitroglycerin, they had to blast through the mountains in a very dangerous manner.

Explanation:

its b they wanted to be able to expand their trade..