The Big Boy, part 8: A long story about long legs!

Posted by John E. Bush
on Sunday, March 31, 2019

 When a person first looks at a Big Boy the initial impression is that it is plainly and simply HUGE

But there’s more than sheer size going on here.  Big Boy also looks good, and this is true both overall and in the details.  In fact, if you’ll permit me a little fun of the sort common to railroaders in steam days, I’ll assert that Big Boy surely had herself a great set of legs!!

Legs? Yup!  To steam-era railroaders locomotive driving wheels were often referred to as an engine’s “legs.”

Articulated steam locomotives were slow, plodding power……Well, or so it had long been considered by railroaders.  While in the 20-odd years following the 1904 construction of Baltimore & Ohio Mallet 0-6-6-0 “Old Maude” there were a few notable exceptions, it would be difficult to argue against what had long been the prevailing thinking surrounding articulated locomotives designed for freight use:  They were slow, period.  From the earliest days following their introduction in the United States articulated locomotives had, by necessity, been distinctly restricted in speed.

In a sizable degree the slow speed resulted from a decision to the use of the “compound” principle of steam expansion.  Without doubt, compound expansion had decided economic benefits.  Think of it as using the same steam to effect work twice……or in the case of this discussion to use the steam again because it is first introduced into the high-pressure main cylinders at boiler pressure; once used in the main cylinder it is exhausted into piping which leads to the low-pressure cylinders on the front engine where it is used again and then exhausted through the stack.  That resulted in serious savings in both water and fuel.  The decision to use compound expansion also brought significant disadvantages.  Now, this blog isn’t intended to be about compound steam locomotives, so if you have interest there are myriad writings available on this principle and its application to locomotives.  However, it is a vital component of any discussion about the development of articulated locomotives and, in this particular case, about Big Boy’s legs on account of problems forced by the decision to utilize compound expansion.

As the accompanying photo will make clear, the compound articulated freight locomotive employed comparatively massive front cylinders.  The huge low-pressure cylinders would use the steam for the second time.  However, having been exhausted from the high pressure cylinders on the rear engine and passed to the front engine, the steam was no longer anywhere near boiler pressure.  It was vital that the designing engineers arrive at nearly equal power for both front & rear engines.  To accomplish this the cylinders for the front engine had to be approximately 1.6 times the size of the high-pressure cylinders.  The result was very heavy pistons and other reciprocating parts on the front engine.  Allowed to run at greater than designed or company-mandated maximum speeds, these engines would wreak havoc on roadbeds and themselves.  They were OK when working upgrade, where speeds rarely exceeded 25 mph, but that speed was not far below their practical maximum, and they were very hard on themselves even when not being pushed to extra-high speeds.

Before long the railroads and builders of articulated locomotives began to make use of simple (or sometimes seen as “single”) expansion steam cylinders of the same size on both engines.  The speed restrictions found with the compounds might have been expected to have been removed, and so some extent they were raised.  However, when simple expansion articulateds began to see regular design and construction in the mid-1920s their use was generally restricted to slow, curving, steep, and challenging operating divisions, which tended to further reduce what today we would consider impossibly slowly moving traffic.

Another major reason early articulated freight engines were slow was found in their short legs.  Driving wheel diameters were generally in the 55-63 inch range.  Prior to about 1920, the problems encountered by the mechanical engineers in attempting to cope with the always present need to increase power and speed had been made worse by the weights of pistons, main and side rods, and other related components.  The amount of weight needed to even reasonably counter-balance the forces created by these heavy parts was often found to be greater than could be contained within the usual method of the placing of weights in driving wheel center castings.  This was especially true of engines with driving wheel diameters of 60 inches and less.  Designers found themselves adding auxiliary balance weights to main driving axles and taking other similar steps.

Wheel diameters of 63 inches were less susceptible to this problem, but at this point in time most articulateds were still being built with driving wheel diameters of 57-60 inches, so speeds remained restricted, even when simple expansion was used with the attendant ability to utilize the same sized cylinders all around.

Serious metallurgical advancements had been realized at an ever-accelerating pace for several decades.  As these new alloy steels developed railroad mechanical engineers began to take advantage of their decided benefits, not least of which was significantly less weight.  Like most new things these alloys were expensive and only the large roads could afford to make extensive use of them in new power.  Initially most new engines made use of these special alloys in axles and springing materials.  As has been experienced ever since railroading began, what works in other industries doesn’t necessarily work on the railroad, or at least on moving trains.  It was pretty quickly determined that while far stronger than had been possible in the past, the early efforts in making main & side rods, piston rods, axles and other heavy service and highly strained parts were overly brittle and had an unacceptable tendency toward breakage.  Dangerous stuff, that!  But soon newer alloys were far more forgiving.

Union Pacific’s mechanical engineering staff was fully involved in the advancement of steels for locomotive use.  When constructed in 1917 and 1922, respectively, the first of UP’s 2-10-2 and 4-8-2 classes were touted in trade press of their extensive use of lightweight steels, and in fact the 4-8-2 utilized more than any previous design to that point in time.  So UP was at the forefront both in the knowledge and in use of these new materials.  At the same time the nation’s railroads were under unceasing pressure to increase train speeds in order to remain competitive.

Yet when UP’s mechanical engineers began working with engineers at Alco in 1917 during the design phase of an order for new 2-8-8-0s they had together determined that the anticipated operating parameters were such that it made good sense to utilize compound steam expansion and employ driving wheels of 57 inches diameter.  Were they thinking backwards?  Hardly, they knew exactly what they were doing.  However, operating conditions were changing rapidly.  WWI also had massive effect.  In fact, had it not been for the Great Depression I am convinced that the later conversion of all UP 2-8-8-0s to simple expansion (including a bunch of other related improvements) would have taken place at least five years before it actually did, because it was soon found that these engines were forming a significant part of several operating bottlenecks……one of which, the Ogden-Green River section, included the Wahsatch grade, was so stubborn it ultimately resulted in construction of the 4-8-8-4s.

Thinking about the slow 2-8-8-0s, such as No. 3618 at Omaha shops, (photo from John Bush collection) forming a major part of the problem that encompassed the Wahsatch and other areas in which these engines operated, and keeping in mind their 57-inch diameter driving wheels, can you tell me what Union Pacific and Alco did to remedy the situation? The answer was 67-inch driving wheels of the 4-12-2s (combined with the advantages brought by the use of 3-cylinder drive) which were designed to move the same tonnage upgrade as the 2-8-8-0s but at twice the average speed, and “haul a mile of freight at passenger train speeds.”  The first of these, No. 9000, was a one-off construction and initially expected to be something of an experiment. It proved so immensely and immediately successful that before trials had progressed two weeks a further 14 engines were ordered.

Think of it, drivers 10 inches larger in diameter than the 2-8-8-0 for a freight engine.  Almost 20 percent larger.

Let’s pause a moment and reflect on some conditions which had a lot to do with what was going on between the last of UP’s articulated power prior to the 4-6-6-4s, and the arrival of the 4-12-2s (including subsequent orders of these engines through 1930).  The main factor was the Depression.  In the period after 1925 it had been expected economic conditions would continue to boom.  But on Black Friday in October 1929, the stock market crashed and as conditions continued to tighten, UP began rapidly storing power in long lines across the system.  Rather than use the lightest power the railroad could run and still get the traffic over the road it was actually the latest and most efficient power which was utilized to the fullest.  In the case of the most of the main lines this was by and large the 9000s.  At that point there was, as far as UP was concerned at least, absolutely no reason to be thinking about more new power and most assuredly management wasn’t looking at articulateds.

However, despite economic conditions locomotive design continued to evolve and some other roads were in more or less greater need of new power.  In the 1934-35 period some simple expansion 2-6-6-4s were constructed with driving wheels of greater than 63-inch diameter, though neither of the roads purchasing them was anything like Union Pacific in their ability to make long runs at speed.  And so, international strife began to result in growing use of American industry to feed, clothe, and arm much of the rest of the world in the middle of the 1930s.  Union Pacific began considering the question of new power to handle increasing traffic, and to do so without spending money to shop older power which was by comparison far less efficient at best.  This was the same reason the 9000s moved most of the traffic since the stock market crash, because they could move more tonnage, faster, on less fuel than the older power.  Thus, it made sense to consider spending huge sums of money on new engines, even when the Depression was still gripping the nation in significant ways.

Much of the rest of this story is well-known history.  In 1935 UP’s locomotive mechanical engineering staff returned to their peers at Alco and together concluded that a new wheel arrangement could allow the railroad to maintain or possibly even slightly increase the speeds being made by the 4-12-2s, but to also do so with the advantages brought by articulation, and by smaller cylinders which provided superior high-speed efficiency, and a boiler made more efficient through employment of higher pressure.  These and other advancements formed important elements of the new 4-6-6-4s which soon received the name “Challenger” and were utilized to every extent possible in company and industry advertising.  This was much in keeping with a national effort aimed at energizing public economic enthusiasm as the country dragged itself out of the Depression.

When the Challengers were designed the mechanical engineers planned for their use in passenger service on difficult operating divisions.  So it’s possible that no one was surprised when they were designed with what was then one of the tallest driving wheels utilized on any American articulated.  In fact, across the nation quite a few roads utilized passenger power with the same 69-inch driving wheel diameter, but here was an engine in which passenger use was considered coincident to the movement of freight at the highest speeds obtainable. They were dual-service for sure, but primarily freight power and high-wheeled freight power at that.

However, this isn’t the case with the Big Boy. They were intended from the outset as freight power.  Sure, they were constructed with full steam heating equipment in order that they could be utilized in passenger service if the need arose (so were the 4-12-2s for that matter), and as we know, there definitely was some troop train usage, but limited passenger train utilization was the extent thinking by the design teams.  So, when looking at Big Boy, I think it’s fun to measure the locomotive’s appearance against the majority of other period big articulated steam power.  Check out their legs, so to speak.  I’m talking power built from about 1935 onward, not so much older engines.  Look at SP’s later ACs, the Duluth Missable & Iron Range and B&O 2-8-8-4s, the Western Pacific 2-8-8-2s and most of the others of the period.  Most have considerably shorter legs.  Big Boy looks more the dancer, less the football lineman.  Big Boy is larger than they are, heavier, longer.  But those 68 inch driving wheels infuse Big Boy with an aura of speed and agility the others don’t seem to possess as far as my own eye is concerned.

Certainly, as we know, tall driving wheels and their use on articulateds was not limited to UP.  Norfolk & Western, Chesapeake & Ohio, Virginian, and others come quickly to mind, and those were certainly great engines, make no mistake about that.  I am not trying to state here that Big Boy is anything better -- only that Big Boy is different.  As far as I’m concerned, the 4000s harbor a race horse or open-wheel racing car look which the others don’t show me.  A great example is the wonderful Allegheny class, a locomotive that is hugely powerful with looks to match. They have a muscle-builder appearance and wonderful it is (so do the 9000s as far as I’m concerned).  But unlike the 4000s, I struggle to think of the C&O/VGN engines flying along.  With Big Boy, I have always found myself imagining one passing me at track speed someplace in the vastness of Wyoming or some other similar high-speed situation.  Not the Allegheny, not the others either.  Big Boy was almost unique among the truly massive articulated freight designs in having been laid out with the understanding that not only was it going to be slogging up the Wahsatch at 25 mph or less, but it was also going to be making track speed to Green River or other locations once grades were surmounted.  Instead of being designed to plod, Big Boy was designed not only to be able to run fast but to do so for long distances.  I don’t believe this had ever before happened in an articulated engine designed almost exclusively for freight use.

Take a few moments to find a nice broadside photo of a Big Boy.  Give it a long look, take it all in and then move in tight.  The 68-inch Book driving wheels (working in concert with roller bearing driving boxes, Alco engineer Jerry Blunt’s sophisticated spring suspension system and state of the art side rod and valve gear designs) permit what can only be said to be truly high-speed operation for such a gigantic machine and do so over long distances and do so while riding very much like a passenger car.  A true feat of engineering.

If you’re fortunate enough to be near when 4014 returns to service in 2019 or during one of her later trips, or if you find yourself standing near one of the seven other Big Boys in their display locations, or even if you’re looking at Big Boy photos with steam friends and the moment presents itself, you might think about having some fun by loudly announcing to anyone within ear shot, ”Just look at those legs!!”

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