Neither of the following sources is new on line, but they're both new to me, and perhaps to you.
The EH.net Encyclopedia of Economic and Business History, part of the Economic History Association, has more than 140 articles by good authorities writing on topics they know about, including agriculture, apprentices, banking, bankruptcy, the 1893 depression, the Dust Bowl, Freedmen's Bureau, fur trade, Gold Rush, indentured servitude, insurance, the Confederate economy, the Revolutionary War, slavery, turnpikes, and toll roads. It's comforting to know that if you need a history of the US carpet industry, it's here. Not to mention fire insurance, or Sweden's economy from 1800.
The University of Missouri at St. Louis has the American Railroad Journal, a weekly, issues 1832-1857, 1865-1878, 1887, 1888, and 1890-1900. (Hat tip: Internet Scout Report.) It's well presented and easy to navigate and search, but has little descriptive material about the site or the gaps in holdings. The periodical is every word searchable and it has unpredictable amounts of information on plank roads and canals as well as business and technical arcana about railroads in general as they were growing up. (Other sites, including HathiTrust Digital Library, have some issues -- and some that UMSL lacks -- but are more difficult to navigate, and seem to have included different magazines under this one title.)
Even genealogists with one-track minds will find material here. The magazine, usually based in New York City, included marriage and death notices in many of its 1833 issues. Its title changed over time, beginning as Rail-Road Journal (January 1832), and changing variously to American Railroad Journal and Advocate of Internal Improvements (January 1837), American Railroad Journal and Mechanics' Magazine (January 1842), American Railroad Journal: Steam Navigation, Commerce, Mining, Manufactures (January 1853), The Railroad and Engineering Journal (January 1887: "the oldest railroad paper in the world"), American Engineer and Railroad Journal (January 1900), and more.
What I have seen indicates a closer focus on railroad business and stocks and bonds than on those who worked in the business, but there is a great deal to explore here -- and a wealth of as-it-happened information and speculation that is often painted over in large-scale histories. And you've gotta love the cute woodcut mastheads from the early days!
Robert Whaples, editor, "Encyclopedia -- Custom," EH.net (http://eh.net/encyclopedia/ : accessed 10 August 2012).
"American Railroad Journal, Full Text, 1832-1900," John W. Barriger III National Railroad Library, St. Louis Mercantile Library, University of Missouri - St. Louis, University of Missouri Digital Library (http://digital.library.umsystem.edu/ : accessed 10 August 2012).
Harold Henderson, "Railroads, Economic History, and Your Ancestors," Midwestern Microhistory: A Genealogy Blog, posted 12 August 2012 (http://midwesternmicrohistory.blogspot.com : accessed [access date]). [Please feel free to link to the specific post if you prefer.]