4-12-2 "Union Pacific" Type Locomotives

In the mid 1920s many western railroads were using the 2-10-2 and 2-8-8-0 type locomotives for mainline freight trains. These locomotives were getting the job done but were limited to about 20 mph. The Union Pacific Railroad experimented with a three-cylinder 4-10-2 in 1925 and actually bought ten of them. These 4-10-2 locomotives were capable of slightly greater speed while being capable of pulling about 20% more tonnage.

The UP wanted more and approached the American Locomotive Company with a design for a 4-12-2 and received the first of this new design in 1926. The locomotive had more power and rode very well at higher speeds. With the arrival of this new wheel arrangement, came the need for a new name. The 4-12-2 was dubbed the 'Union Pacific' type and it namesake railroad was the only North American railroad to use it.

The American Locomotive Company built eighty-eight of these fast freight locomotives for the Union Pacific Railroad. They were three-cylinder locomotives with the third cylinder located in the center of the steam chest. The third cylinder drove an eccentric in the axle of the second pair of drivers. The fourth set of drivers were flangeless, but it was later determined that this was not necessary and a small flange was added to these drivers.

The UP ran these locomotives at 50 mph hauling 120 car trains.

Builders of 4-12-2 "Union Pacific" Type Locomotives (by Richard Duley)

Number of Locomotives by Builder
RailroadALCOBaldwinLimaOtherFrom Other RRs
Union Pacific88
Total88


Number of 4-12-2 "Union Pacific" Type Locomotives Built by Year (by Richard Duley)

RailroadYearTotals
19261927192819291930
UP1523252588
Totals:1523252588

4-12-2 "Union Pacific" Type Locomotive Details

UP 9000 Geometry

The UP 9000s had two outside 27"x32" cylinders driving the third set of drive wheels and a third 27"x31" cylinder in the center driving the second axle. The 9000s had 67 inch drivers. They were constructed with "blind" driver tires on the third and fourth axles, that is, without the usual flanges, in an effort to promote easy passage through tight curves. The blind drivers were found to be unnecessary as tests of the first 4-12-2 proved the usefulness of the lateral motion devices fitted to the first and sixth driving axles. There was an attempt to see if they could negotiate the Oregon Short Line in the Mountains of Oregon. The test failed and they were returned to the prairies of Nebraska for which they were designed in the first place.

There is an extremely accurately detailed model of the UP 4-12-2 available for general viewing at the Schenectady Museum, in Schenectady, NY. The model is in a section of the museum called the Schenectady Heritage Area. The model provides an excellent view of the lateral motion devices fitted to the first and last driving axles, spring rigging, main and side rod designs, cylinder configurations, valve gear arrangements, etc. The model is covered by a clear plexi-glass cover which allows for viewing but no touching.

1st Axle Clearance Crank?

There has been some disagreement on whether or not the first drive axle on this class of locomotives has a 'clearance crank'. The typical configuration for a three-cylinder steam locomotives is to have a cranked first axle so that clearance is provided to the connecting rod that is connected to the second set of drive wheels. The Guide to North American Steam Locomotives by George Drury states that the first axle is cranked. I have also received e-mail which stated that the first axle has a crank. However, I have been in contact with John E. Bush who was the co-author of an excellent reference book on these locomotives (Volume 2 of The Union Pacific Type by Kratville and Bush). This book has several photos which show quite clearly that it does not have a 'clearance crank' on the first axle. Instead, it explains, the builders added 18 inches to the distance between the first two axle centers, thus precluding the need for a 'clearance crank' on the first driving axle. This has been verified by Barry Koeb who is the R&LHS member responsible for the UP 9000 who has been inside the frame of the 9000. I am now convinced that the first axle on this class of locomotives did not have a crank. The last time I was in LA, I tried to look under 9000 and photograph the first axle. However, the front of this locomotive is so tightly packed that I could not see the front axle let alone photograph it.

In summary, the 9000's designers wanted to avoid having to put a crank in the axle of driver number 1. So they did the following things:

Driver "Quartering" and Valve Gear

The standard practice for two-cylinder steam locomotives is to "quarter" the drivers. In other words, configure the main crank pins 90° apart on either side of each axle. With a three-cylinder steam locomotive, the main driver crank pins would be placed 120° apart provided that each of the three cylinders was in a flat plane.

Because ALCO placed the center cylinder on the UP 9000s at an incline of 9½°, some have said that it would be necessary to set the crank pins at 120°-129½°-110½° to preserve the torque distribution and to keep the system balanced. This was not the case with the 9000s. The one inch shorter stroke of the center cylinder and valve event (timing) dimensional settings allowed for the center cylinder inclination.

This assertion has been confirmed by Joe Davenport of UP's Mechanical Department as well as by certain ALCO documents. Additionally, ALCO had repeatedly referred to the use of 120° crank spacing in their literature devoted to their effort toward sales of three-cylinder power. Finally, a Union Pacific drawing, number 112CA27929, dated May 6, 1941, titled "Axle--Key Location" confirmed the 120° spacing. The intention of this drawing was to provide machinists with data permitting proper location of the keyways necessary to affix driving wheel center castings to their respective axles. Sure enough, all three pins are clearly displayed and specifically noted as 120 degrees apart. Of course, only the 2nd main axle actually had three pins since that is where the inside cylinder was connected. Pins on the other five axles were also of necessity spaced at 120° and thus drawings for these engines can take your eyes a moment to adjust to given the usual propensity for pin spacing at 90 degrees with two-cylinder engines.

It is suspected that one of the reasons the Baldwin #60000 used a 90°-135°-135° configuration had to do with it being a compound locomotive, with the center cylinder receiving high-pressure boiler steam, and the outer cylinders receiving the low-pressure steam from the center cylinder. And according to John Bush, the locomotive had "a very interesting" outside valve gear design. The original valve gear used a conjugated assembly that synthesized inner cylinder valve gear motion from the outer valve gear on either side of the locomotive.

UP 9000 driver "quartering" information graciously provided by John E. Bush.

"Loping" Exhaust

Many have pointed out the "loping" exhaust rhythm one hears in recordings of the 9000 class. The sound may be the result of a number of factors: When released from shops with running gear trammed "to dimension" and valves set correctly, a 9000 had a very even 1-2-3, 4-5-6 beat. All of the main dimensions were the same for the outside and inside cylinders and valves (except that the stroke of the inside cylinder was one inch shorter) and when everything was right they were square like any engine, only with six exhausts per revolution. It was frequently heard that one exhaust port or another would cause a louder chug than the others, but the resulting 1-2-3,4-5-6 did not equate to being out of time or "out of square". Like other power, wear and "lost motion" developed as miles grew, and in particular, maintenance forces allowed attention to the inside main rod to languish! If you've heard Howard Fogg's recordings of 9009 on his great album "The Big Steam" and think thats what a 4-12-2 regularly sounded like please consider it as virtually totally unrepresentative of their sound. When Howard made his recording, the 9009 was horribly out of time. When badly out of dimension, engines can begin working against themselves and 9009 was probably beating itself to death.

UP 9000 exhaust information graciously provided by John E. Bush.

Other Long, Rigid Wheelbase Steam Locomotives

The UP 9000s had the longest rigid wheelbase of all steam locomotives in the United States. However, there were a few other countries with almost equally impressive locomotives. Bulgaria and two groups of 2-12-4Ts, Indonesia had a few 2-12-2Ts. Russia built one 4-14-4 (classified as a 2-7-2). It made one demonstration run during which it tore the track apart -- it was stored and later scrapped. It was designed shortly after a group of thirty young Soviet engineers toured the USA in 1930 and 1931. They saw the last of the Union Pacific 4-12-2s being built by Alco's Dunkirk New York shop. Steam locomotive historians consider the Soviet 4-14-4 to be an example of the Russian's trying to outdo the Americanskiies. The Russians learned many useful things during their tour, and did develop a successful 2-10-2 freight engine and a 2-8-4 passenger locomotive using ideas they picked up from Alco and Baldwin.

4-14-4 Photos:

USA and Canadian Railroads that used 4-12-2 "Union Pacific" Type Locomotives

Surviving Examples of 4-12-2 "Union Pacific" Type Locomotives

No.ClassF.M. WhyteGaugeRailroad LineLocationStatusBuilder InformationNotes
900090004-12-24'-8½"UP Los Angeles Co. Fairplex, Pomona, CAdisplayALCO (Brooks) #66544, 1926 Three cylinder

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