Building on American Indians' footpaths, the Oregon Trail became the main route west to the Pacific Northwest and, after Astor's Pacific Fur Company opened the trail through the Rockies at South Pass in 1812 and mountain men guided the first wagon train over it in 1841, it served until the Transcontinental Railroad was completed in 1869 as the way west for emigrants, Mormons fleeing persecution, California gold seekers, and later the Pony Express. As many as 500,000 men, women, and children traveled some 2,000 miles across deserts, plains, and mountains. After gold was discovered in California in 1848, nearly 75,000 "49'ers" passed through this valley by 1850, and numerous cutoffs were developed, some pioneered by California emigrants and now known as the California Trail. Brigham Young led the first Mormon wagon train west along the trail in 1846-47, following it to Fort Bridger before taking the Hastings Cutoff into the Salt Lake Valley, then part of Mexico until changed by the Mexican-American War. Beginning in 1860, the California Overland Mail and Pikes Peak Express gave rise to the Pony Express, whose riders carried mail for 18 months over 1600 miles from St. Joseph, Missouri, to Sacramento, California, in 15 days. Wyoming offered the only route across the West with the three essentials for travel: good water every day, dependable grass, and a passable grade to and through the Rocky Mountains at South Pass.