The Mountain Type, with a 4-8-2 wheel arrangement, was a predecessor steam locomotive design of the later highly successful Northern Type. The new type debuted in 1911 for the Chesapeake & Ohio Railway to replace the use of Pacifics, which could no longer effectively handle the increased weight of freight and passenger cars, particularly over steep grades (this is also why the new type received the name Mountains from the rugged Appalachian range the C&O operated through).
The Mountain Type was originally built by the American Locomotive Company (Alco) for the C&O but by the time production ended on the design, Baldwin and Lima Locomotive Works had also contributed to the over 2,000, in total, built for U.S. carriers. While the Pennsylvania and New York Central would come to own the most Mountain Types several other Class I systems also rostered 4-8-2s by either purchasing from a manufacturer or simply building their own. For a look at some of the railroads which rostered Mountains, and their designated classes, please have a look at the table below (Please note that the class listing here is very general as many railroads broke the locomotive down into further designations such as H-1a, H-1b, H-1c, etc. Also, not all railroads are listed that owned Mountains):
· Baltimore & Ohio: Class T3 44
· Burlington: Class B-1 21
· Chesapeake & Ohio: Class J-1/J-2 10
· Florida East Coast: No Designated Class 90
· Illinois Central: No Designated Class 136
· Lackawanna: No Designated Class 45
· Missouri Pacific: Class MT 33
· New Haven: Class R (1-3) 70
· New York Central: Class L (1-4) 600
· Norfolk & Western: Class K (1-3) 48
· Pennsylvania: Class M1 301
· Rock Island: Class M50 62
· Santa Fe: No Designated Class 51
· Southern Pacific: Class Mt (1-5) 83
· Southern Railway: Class Ts 58
· Union Pacific: Class MT 60
Today, several of these beautiful locomotives survive including many from the St. Louis-San Francisco Railway (the Frisco) and at least one is still in operating condition, Canadian National #6060, owned by the Rocky Mountain Rail Society based in Calgary, Alberta.
For more information on the Mountain Type steam locomotive consider Guide to North American Steam Locomotives by author George Drury, which includes nearly 500 pages of information on virtually all of the steam locomotive wheel arrangements and designs (including streamlined steamers) ever developed. The book is a great resource on steam locomotives and a fine reference tool; you should find it very useful.
Also, consider the book American Steam Locomotives from author Brian Solomon. While this publication does not include quite as much technical data as Guide to North American Steam Locomotives it is still a very good resource with lots of information and best of all, is loaded with photographs! If you're interested in perhaps purchasing these books please visit The Railroad Diamond by clicking the tab in the menu to your left marked "TRD Store".