What was the most significant obstacle in building the Transcontinental Railroad?

The most significant obstacle in building the Transcontinental Railroad was the challenging and varied terrain across the United States, particularly the Sierra Nevada mountains and the Great Plains. In addition to the physical landscape, harsh weather conditions, financial constraints, and labor shortages also posed major challenges. Conflicts with Native American tribes over land rights further complicated construction efforts, leading to delays and increased tensions. Ultimately, overcoming these obstacles required innovative engineering solutions and the collaboration of multiple railroad companies.