Missouri—Gateway to the West—became the pathway for half a million emigrants seeking a better life. Independence, Westport, and St. Joseph developed into bustling outfitting posts for the three major trails  headed west: Oregon, California, and Santa Fe. The first true emigrant train left Missouri in 1841 guided by Thomas Fitzpatrick, but the first major migration to Oregon came in 1843.
"Caravans left Independence in the early spring on the road to West Port. The California, Oregon and Santa Fe Trails were, it should be noted, one and the same along this route. It was after they left West Port that they divided and took on their respective, singular names" (Lee, Fred. "The Trails and How They Passed through Westport." Westport (May 1980):6.).
"Of those who set out in the spring of 1850 was a large party among whose members was Rev. Robert James, the father then of two little prattling boys, who afterwards became the noted bandits, Frank and Jesse. En route Mr. James wrote one letter to the Liberty Tribune, which is still preserved in the files of that paper. Not long after his arrival in California, Rev. James died" (History of Clay and Platte Counties, 1885: page 153). [MVSC 977.816 C62-2]
Robert James  wrote from Fort Laramie, May 18, 1850:
We hold tent worship nightly in some one of our tents and every Sabbath evening prayer meetings. Many of the emigrants are in great distress. Some are going badly prepared for the trip. One man is going through with a wheel barrow, who out travels our mule teams … May the blessings of God rest on us until we shall have completed our expedition and return home safely to our families and friends.
This letter is now in the Missouri Valley Special Collections Liberty (Mo.) Tribune Papers (SC34) . Another letter written to the Liberty Tribune, this one by N. P. Carpenter from Sacramento City, California, Dec. 9, 1849, advises only "able-bodied" men to venture on the trail.
I would not advise any man, who is engaged in business that will yield him a decent support, to come and risk his fortunes here---A young, stout, able-bodied man, who is willing to undergo a great deal of hard labor, had, perhaps, better come. All lazy people had better stay away.
Learn all about traveling overland in a covered wagon on Packing Up and Heading West: The History of Covered Wagons .