What is the difference between the union pacific railroad and the central pacific railroad?

The Union Pacific Railroad and the Central Pacific Railroad were two key components of the First Transcontinental Railroad in the United States, completed in 1869. The Union Pacific, starting from the east, primarily expanded westward from Omaha, Nebraska, while the Central Pacific began in Sacramento, California, and built eastward. The two lines met at Promontory Summit, Utah. Additionally, the Central Pacific faced significant challenges, including the Sierra Nevada mountains, which necessitated extensive tunneling and engineering efforts.