There were a number of events that increased interest in a route to California. Probably the most important was the acquisition of California in the Mexican-American War, and the immediately following California Gold Rush. The Civil War added urgency to the transcontinental Railroad, and was very important as well.
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building of the transcontinental railroad - disappearance of the frontier
The California Gold rush of 1849.
The Union Pacific and Central Pacific built the transcontinental railroad and when it was finished, a golden spike was nailed when it was finished to celebrate the event.
The Mexican War and the debate over the new territories such as California.
The driving event was the westward expansion of U.S. territory, esp. in connection with the Mexican War (the product of the annexation of Texas). How the territories were to be organized - whether open to slavery or not - became a burning issue. The territorial issue was intensified at the end of the decade, by California's growth (aided immensely by the Gold Rush) and the issue of building a transcontinental railroad through the territories to link east & west.