The classifications given by railroads to their steam locomotive fleets were as varied as the machines themselves with several different designations within a particular class! Western Pacific Railroad steam locomotives were likewise distinguished by a wide range of numerical classes (a bit from the norm when most Class I systems distinguished their varying steam types by letters). In any event, the information here is most certainly not a complete, all-time listing of the WP's steam fleet and also is merely meant to list the general types of steam locomotives operated by the railroad.
The Western Pacific was perhaps a railroad that should have never been built and had Collis P. Huntington and the railroad magnates of California had their way it certainly would not have! The WP was the longtime dream of Arthur Keddie who wanted to construct a railroad through the Feather River Canyon of Northern California. For nearly 80 years the WP moved freight through its well engineered, albeit high maintenance, main line between San Francisco/Oakland and Salt Lake City although its small size (just over 1,000 of total mileage) and being surrounded by giants ultimately cost it its independence (the railroad, however, was a tenacious fighter).
For its small size, however, the railroad was well known for a number of things including being one component of the highly successful California Zephyr passenger train and its Keddie Wye at Keddie, CA (which features a magnificent split-bridge where its Inside Gateway, a line that diverged from its Salt Lake City main line, connected to Bieber, California and a link with the Great Northern).
The Western Pacific did not operate a large steam locomotive but nevertheless did roster some massive designs including 2-6-6-2s, 2-8-8-2s, and 4-6-6-4 Challengers. Today, no large articulated models survive but other still do.
Class 1, 20, 124
These classes designated WP's fleet of 2-8-0 Consolidations.
Class 71, 121
These classes designated WP's fleet of 4-6-0 Ten-Wheelers.
Class 151, 163
These classes designated WP's fleet of 0-6-0 switchers.
Class 171
The WP's Class 171 designated its only roster of 4-8-2 Mountains.
Class 201
The WP's Class 201 designated its only roster of 2-6-6-2 heavy articulateds.
Class 251
The WP's Class 251 designated its only roster of 2-8-8-2 heavy articulateds.
Class 301, 306, 321, 322, 327
These classes designated WP's fleet of 2-8-2 Mikados.
Class 401
The WP's Class 401 designated its only roster of 4-6-6-4 Challengers.
Class 481
The WP's Class 481 designated its only roster of 4-8-4 Northerns.
It's jointly operated California Zephyr with the Denver & Rio Grande Western and Chicago, Burlington & Quincy also lives on under Amtrak, albeit following a bit of a different route than the original. So for WP fans, all is not lost as UP locomotives still ply the Feather River Canyon and photographers can still catch freights on the famous Keddie Wye.
For more reading about steam locomotives like those operated by the WP consider Perfecting the American Steam Locomotive by author J. Parker Lamb. As the name implies the book details the earliest history of steam engine technology, even before it was used in railroad applications. His book later explores the development of steam locomotive technology in the United States from the 19th through the 20th centuries, covering not only the most popular steam locomotive designs but also the most successful manufactures to build them. The book has received excellent reviews and 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 Lamb's book, Perfecting the American Steam Locomotive, 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 either (or both) of these books please visit the links below which will take you to ordering information through Amazon.com, the trusted online shopping network.