Southern Pacific SP-2 locomotive 5021. The 4-10-2 locomotive is now owned by the Railway and Locomotive Historical Society of Southern California.

(University of Southern California Library/California Historical Society, CC BY 3.0 <>, via Wikimedia Commons)



Under the Whyte notation for the classification of steam locomotives by wheel arrangement, 4-10-2 represents the arrangement of four leading wheels, ten powered and coupled driving wheels and two trailing wheels. In South Africa, where the wheel arrangement was first used, the type was known as a Reid Tenwheeler. In the United States of America it was known as a Southern Pacific on the Southern Pacific Railroad and as an Overland on the Union Pacific Railroad.

In the United States, a simple expansion (simplex) version of the type was used only on the Southern Pacific and Union Pacific Railroads. Baldwin Locomotive Works built an experimental compound expansion 4-10-2 in 1926, but since the weight and length of this engine was too much for all but the heaviest and straightest track and compound steam locomotives had already lost favor on United States railroads, its demonstration runs failed to generate interest and no more were produced.


A diagram of the 4-10-2 Wheel Arrangement. Front of locomotive on left.

(Gwernol, Public domain, via Wikimedia Commons)


SP 5021 while it was stored on the Santa Fe at San Bernardino, CA in February 1972. It's now at the

Rail Giants exhibit in the LA County Fairgrounds in Pomona CA. A Roger Puta Photograph

(Marty Bernard, Public domain, via Wikimedia Commons)



In the United States, the type was used only on the Southern Pacific Railroad (SP), which called it the Southern Pacific, and the Union Pacific Railroad (UP), which called it the Overland after their corporate sobriquet, The Overland Route. Only sixty locomotives of this wheel arrangement were built for domestic service and all but one were constructed as simplex three-cylinder engines.

In 1925, the SP placed an order for sixteen 4-10-2 locomotives with the American Locomotive Company (ALCO) and, later in the same year, the UP ordered one. The first SP locomotive, no. 5000, was completed in April 1925, while the UP locomotive, no. 8000, was completed the following month. Within a few months, the SP ordered more of these engines and built up a fleet of 49. The UP, on the other hand, waited thirteen months before repeating orders and establishing a fleet of ten 4-10-2 locomotives. All 59 were simplex locomotives and were built by ALCO.

In 1926, Baldwin Locomotive Works constructed an experimental demonstrator, the Baldwin 60000, which was a three-cylinder compound locomotive, the only 4-10-2 so constructed. This engine used high-pressure steam in the inside cylinder and then exhausted that steam into the two low-pressure outside cylinders. It also had a water tube boiler, one of a very few locomotives so equipped in the United States. As technologically innovative as Baldwin's 4-10-2 was, however, it was outmoded when built since compound steam locomotives had already lost favor in United States railroading.

It was found that the 4-10-2 type ran better and rode smoother than the 2-10-2 type from which it had evolved. The third cylinder in the center of the cylinder saddle sloped down at a 9½ degree angle to a crank on the second drivers' axle, while the two outside rods connected to the third drivers. The three-cylinder feature on these locomotives gave them a distinctive sound at work, described as a "hop, skip and jump rhythm".

While the SP engines could operate only on relatively straight and heavily built mainlines, their long service lives of between 28 and 30 years proved that they were good locomotives. The most serious mishap was when, in November 1946, one of the SP 4-10-2 locomotives suffered a boiler explosion which killed four train crew. In addition, the inside cylinder's rod created serious maintenance problems because the floating bushings could fail, which lengthen maintenance down-time since such failures required major valve re-settings that caused delays. It was reported that such failures created such great pounding on the rails that railroad housewives along the line "complained of crockery cracking when a defective 4-10-2 rumbled past their homes".

When the UP became tired of the major mechanical problems associated with three cylinders, it converted its locomotives to two-cylinder locomotives in 1942 and renumbered them 5090 to 5099.

Two locomotives of this type have been preserved:

  • Southern Pacific 5021, on static display at the Los Angeles County Fairgrounds in Pomona, California.
  • The Baldwin 60000 demonstrator, on display at the Franklin Institute Science Museum in Philadelphia, Pennsylvania.



Equivalent classifications
UIC class: 2'E1'
French class: 251
Turkish class: 58
Swiss class: 5/8
Russian class: 2-5-1
First known tank engine version
First use: 1899
Country: Colony of Natal
Locomotive: NGR Class C
Railway: Natal Government Railways
Designer: George William Reid
Builder: Dübs and Company
Evolved from: 4-8-2T
First known tender engine version
First use: 1925
Country: United States of America
Locomotive: SP-2 class
Railway: Southern Pacific Railroad
Designer: American Locomotive Company
Builder: American Locomotive Company
Evolved from: 2-10-2
Benefits: Could handle roughly 20% more tonnage than the 2-10-2 while using around 16% less fuel than the 2-10-2.
Drawbacks: Complex to maintain because of the third cylinder