In 1863, Union Pacific started construction in Sacramento. Eastward with a railroad track. In 1865, Central Pacific started in Omaha. Westward with a railroad track, 18,000 workers, no modern equipment, as we now have. Crossing the Sierras and the Rockies, Indian country and thieves.
Harsh? Yes, very much so.
Winter snow, and all the elements. Six years later they met in Utah.