<Ul> <Li> A northern route roughly along the Missouri River through present - day northern Montana to Oregon Territory . This was considered impractical due to the rough terrain and extensive winter snows . </Li> <Li> A central route following the Platte River in Nebraska through to the South Pass in Wyoming, following most of the Oregon Trail. Snow on this route remained a concern . </Li> <Li> A southern route across Texas, New Mexico Territory, the Sonora desert, connecting to Los Angeles, California . Surveyors found during a 1848 survey that the best route lay south of the border between the United States and Mexico . This was resolved by the Gadsden Purchase in 1853 . </Li> </Ul> <Li> A northern route roughly along the Missouri River through present - day northern Montana to Oregon Territory . This was considered impractical due to the rough terrain and extensive winter snows . </Li> <Li> A central route following the Platte River in Nebraska through to the South Pass in Wyoming, following most of the Oregon Trail. Snow on this route remained a concern . </Li> <Li> A southern route across Texas, New Mexico Territory, the Sonora desert, connecting to Los Angeles, California . Surveyors found during a 1848 survey that the best route lay south of the border between the United States and Mexico . This was resolved by the Gadsden Purchase in 1853 . </Li>

When did the transcontinental railroad start to be built