The correct answer is:
Mountains
The transcontinental railroad was a 1,912-mile continuous railroad line constructed between 1863-869 that connected the existing eastern U.S. rail network at Omaha, Nebraska/Council Bluffs, Iowa with the Pacific coast at the Oakland Long Wharf on San Francisco Bay. It was built in six years almost entirely by hand. The main geographic feature provided the biggest challenge to the central pacific railroad workers during the construction of the transcontinental railroads was tunneling through the Sierra Nevada mountains.
