I would suggest that you try to find and read a copy of Eugene L. Huddleston’s book “The World’s Greatest Steam locomotives”. In this book Mr. Huddleston compares the “Allegheny”, the Class A, and the Challenger. Plus, some other tid-bits!
Many thanks for the reading suggestion, I am ever on the prowl for a new book to expand my collection/knowledge and will certainly look into acquiring a copy of this one! Additonally thank you and others for the responses regarding the Class A 2-6-6-4’s, I do hope I did not come off as disparaging these fantastic locomotives, I wish only to expand my understanding of these fascinating machines.
If I may turn back to my initial question and request your and others knowledge on the subject, I have one standing question in regards to the Z-8 Challengers of the NP and SP&S. As I understand it, the SP&S challengers were converted to burn oil, however kept the gargantuan fireboxes of the original NP design. Would this conversion to oil have any appreciable impact on the performance of the locomotives? What differences might one expect going from burning the incredibly low quality coal used by the NP to burning oil? Any insight into this is greatly appreciated.
Great Northern also owned two Z-6 oil-fired Challengers that were bought from subsidiary Spokane, Portland & Seattle that were numbered 4000 and 4001.
Eventually as it dieselized the Great Northern sold them back to SP&S.
But Great Northern is not typically though of as a Challenger owner but it was in the 4-6-6-4 club!
https://www.deviantart.com/avalanch11/art/Great-Northern-4-6-6-4-4000-Steam-Locomotive-996878578
Comparing the different Challengers may be a bit hard as they are geographically far flung enough that their usage may vary, and what works for the Northern Pacific may not be applicable to the Delaware and Hudson.
I think that there are two ways that you could cut this to be interesting.
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Look at the usage of the UP challengers on the Rio Grande and Clinchfield. If they worked essentially just as well on all three roads then that is a lot of geographical distance and perhaps different usage covered by one loco type and would speak to a universal quality.
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Compare the UP Challengers on the Rio Grande to the Rio Grande Challengers. This compares two the two different Challenger different builders (ALCO and Baldwin) on essentially the same road and conditions.
On a subject change, I feel like I have heard of the Western Maryland Challengers being a cut above. Something to the effect of them being higher horsepower than the others?
Read in a book about the WM, the operating costs of their Challengers was such that the WM parked them several year BEFORE their equipment trusts expired - thus WM found it to their advantage to pay on the equipment trust without using the equipment that the trust was for.
Well, that ain’t good
Although there is likely insufficient operational and performance data to undertake a full comparison of the several Challenger types, at least a relatively simple dimensional comparison may be made.
The original UP Challengers of 1936 were built to a 65 000 lb axle loading limit, and had 386 000 lb on drivers. The grate area was 108 ft² (a number inherited from the 4-12-2). Boiler pressure was 255 lbf/in², cylinder dimensions were 22” x 32”, with 69 inch drivers. Alco and UP may have been concerned about front engine stability at higher operating speeds. So, although a conventional (two-plane) articulation joint was used, it was friction damped on its pitch axis. The pilot truck also had pitch damping. The choice of four-wheel pilot truck was part of this, although the total weight, coupled with the relatively modest weight on drivers imposed by the axle loading limit probably made the use of the four-wheel truck mandatory anyway. Apparently, reduction of rail thrusts was one of UP’s objectives, and it was anyway accustomed to using four-wheel pilot trucks on its fast freight locomotives. To put the UP Challenger adhesive weight into perspective, the N&W 2-6-6-4, as first built in 1936, had 430 000 lb on drivers, suggesting that it was built to an axle loading target or limit of 72 000 lb, more than 10% higher than the UP number.
The following NP Challenger was a derivative of the UP design, with a much larger (152 ft²) grate area to facilitate the burning of rosebud coal. Thus, it was heavier, with an adhesive weight of 435 000 lb, suggesting a target axle loading of 72 500lb. Perhaps to take advantage of this, the cylinder diameter went up to 23”. Boiler pressure was 250 lbf/in². It retained the same pitch damping arrangement as on the UP locomotive. The longer firebox meant a longer engine wheelbase, 61’10” as compared with 59’11”.
The 1938 WP Challenger appeared to have been more-or-less the UP design with some detail changes, and somewhat heavier. Adhesive weight was 416 000 lb. Perhaps the axle loading target was 69 500 lb or thereabouts. Cylinders were the “standard” 22” x 30”, with 70 inch drivers and a 265 lbf/in² boiler pressure. Whether it had the pitch damping arrangement is unknown, but as a clone of the UP original it does seem likely.
The 1940 D&H Challenger was a modernization of the original UP design, retaining the same basic dimensions, including the 108 ft² grate area. The driving axle load target was 68 000 lb, and the adhesive weight was 406 500 lb. That was distributed asymmetrically, 201 000 lb on the rear and 205 500 lb on the front, this to provide nominally equal distribution on a 1.5% upgrade. Boiler pressure was 285 lbf/in², and cylinders were 20½” x 32”. These dimensions appear to reflect the D&H leaning towards higher-than-typical boiler pressures. Mechanically, it had cast frames, something that the earlier Alco Challengers did not, but probably should have had. Also, it had a single-plane articulation joint, which dispensed with the need for pitch axis damping. This type of joint seems to have originated with the N&W 2-6-6-4 in 1936. It might have been a GSC idea, something that took advantage of the extra strength of cast frames.
The second NP Challenger design in 1941 was a little longer than its original, with a total engine wheelbase of 62’2”. The boiler had the same grate area as before, but was slightly longer. Adhesive weight went up to 444 000 lb, suggesting an upward axle loading creep to 74 000 lb. Cylinders were still 23” x 32”, but drivers were 70 inches (with slightly longer driving wheelbase) and boiler pressure was 260 lbf/in². As far as I know, NP retained the two-plane articulation joint. Whether it also retained the pitch axis damping is unknown
The 1942 Clinchfield Challenger was the D&H design with detail changes. A 70 000 lb driving axle load allowed a 420 000 lb adhesive weight. It reverted to the “standard” 22’ x 32” cylinder size with 265 lbf/in² boiler pressure.
But the 1942 UP “Big” Challenger was virtually a complete departure. Rather it seemed to have been more-or-less a three-quarter sized edition of the Big Boy. Grate area was 132 ft², boiler pressure was 280 lbf/in², cylinders were 21” x 32”, and drivers were still 69”. The axle loading target was 67 500 lb (a number which UP appeared to have adopted with the original FEF), and adhesive weight was 404 000 lb. They had cast frames and single-plane articulation joints. Also they had the full Alco-Blunt system of lateral controls, as did the Big Boy, and which UP had adopted with the FEF-2 in 1939.
Baldwin’s first of two Challenger designs was in 1938 for the DRGW. It had a 136.5 ft² grate area, 255 lbf/in² boiler pressure, 23” x 32” cylinders, 70” drivers and 438 000 lb adhesive weight, suggesting a 73 000 lb axle loading target. It does appear that Baldwin was aiming at a somewhat larger locomotive overall than the baseline Alco design. It did have cast frames, but with a conventional two-plane articulation joint.
Baldwin’s second Challenger design for WM in 1941 was somewhat smaller, broadly similar in size to the Alco baseline design. Grate area was 118.8 ft², boiler pressure was 250 lbf/in², cylinders were 22” x 32”, and drivers were 69”. Adhesive weight was 402 000 lb, suggesting an axle loading target of 67 000 lb. It had cast frames with a conventional articulation joint. It did have a different steam distribution system to the DRGW design, though.
Notwithstanding the variations indicated in the foregoing, all of the Challengers had one common dimension, namely 102 inches boiler maximum outside diameter (BMOD).
Note also that later batch builds of a given design sometimes had variations, particularly in respect of weight.
I should be wary of drawing too much from the above very simple comparison, which does not at all address the thermal aspects. Nonetheless, on the basis of adhesive weights alone, it is not difficult to see a possible reason as to why DRGW preferred its own design over the UP “Big” design foisted upon it by the WPB. Simply its own design would have been able to reliably start and accelerate a heavier load on the ruling grade.
UP does seem to have been more concerned about rail stress than most. I suspect that the 67 500 lb axle loading that it allowed for its late steam designs, conservative relative to what other large roads were doing, did not reflect a lighter track and roadbed, but a concern as to what happened at higher sustained speeds, given that rail stresses increase as the square of the speed. Its adoption of the Alco-Blunt lateral controls is another indicator. Possibly the severe winter conditions to which most of its main line was exposed was another factor in its conservatism.
The original Alco UP Challenger was somewhat “shaped” by the fact that at least to a first approximation, it was an articulated derivative of the UP 4-12-2. The latter was built to a nominal axle loading of 59 000 lb, although the trailing truck was over at 60 000 lb. Thus, any likely derivative was almost certain to have an additional carrying axle. The four-wheel pilot truck came into UP freight locomotive practice as a matter of necessity, with the 4-10-2. This was intended to be as much like its 2-10-2 as possible, but the extra weight, and probably the extra overhang, of the three-cylinder assembly, necessitated a four-wheel pilot truck. This was carried over to the 4-12-2, and apparently the UP found it advantageous in general.
Cheers,
Just wanted to say thank you, this was both a highly informative and entertaining read and gives a good understanding of. the differencies and similarities between the various Challenger designs.
If I am remembering correctly, I believe the D&RGW used a handful of the later Alco designed Challengers which were diverted from a UP order to make up for a shortage in motive power on the D&RGW during the war. To your knowledge, how did these UP Challengers fare in service on a different railroad? From what I’ve read the D&RGW wasn’t particularly enamored with these locomotives. Was this due to any inherent flaw in their design, or was it more that the Challengers were delivered in lieu of the FT diesels that many roads were trying to get their hands on around this time? Is there any data or testimony on how these locomotives stacked up to the Baldwin 4-6-6-4’s already in use on the D&RGW?
I have no insight on how they were viewed by the line, but Trains reported at the time that the Rio Grande returned them to the War Assets Administration since they were surplus to their needs with the postwar traffic decline and they didn’t want to purchase them.
Rio Grande had originally wanted to order some additional Baldwin 4-6-6-4’s. The war production board would not do so and diverted some from a UP order. The Alco UP design challengers diverted to the Rio Grande were on lease from the War Production Board. The Rio Grande crews did not like them. From what I remember reading (but I can’t remember the source and was trying to find it) the crews found the pullling power lacking along with the ride qualities compared to the Baldwins. I am trying to remember but it seems like they may have had some issues with the centipede tenders. UP and Rio Grande approaced their usage of challengers a bit differently. UP tended to run higher speeds- Rio Grande slower and longer trains.
Kratville, in his book on the UP Challengers, said the following in respect of the six ‘diverted’ to the DRGW:
‘The Union Pacific applied in late 1942 for the 3975-3999 series, the engines being finally delivered in mid-1943. The road desired more units but that is the most the W.P.B. calculated could be put through Alco in the light orders for other roads.
‘At the same time, the W.P.B. was assisting the Denver and Rio Grande Western gear up for the eventual Pacific Theatre operations transportation shift and added six additional Challengers to the U.P. order. The D.&R.G.W. made it clear that it probably would not want the six units after the war particularly because of having to keep parts in stock for just six locomotives. The Union Pacific did not offer to take them either since by this time Jabelmann was already laying out his own plans to dieselize the road right after the war.’
Thus, it looks as if it were predetermined that the DRGW would not keep these locomotives beyond the end of WWII.
Cheers,
Nominally at least, one might reasonably expect that the much larger firebox volume and grate area of the NP 4-6-6-4, as well as other changes such as in the type of grate used, as compared with the UP original, was intended to compensate for the poorer quality coal used by the NP. Whether in practice it under- or overcompensated is unknown. Possibly it did both, at different points in the power vs speed curve.
With oil firing, grate area is a meaningless parameter. Firebox volume and firebox length appear to be more important. Nonetheless, most oil fired locomotive fireboxes seem to have been designed with possible conversion to coal firing in mind, so that a notional grate area was typically quoted anyway.
Many of the UP Challengers were converted to oil firing. None of the available information suggests that there was any differentiation between the oil-fired and coal-fired versions in terms of load schedules or expected end-to-end speed performance. A reasonable inference is that in oil-fired form, the firebox and draughting details were chosen generally to match the coal-fired form in overall performance, notwithstanding the higher heating value of the fuel oil, say around 18 000 BTU/lb as compared with the 11 800 BTU/lb of the coal used by the UP.
Possibly the SPS oil-burning Challenger were setup to more-or-less match those on the NP. The NP rosebud coal was said to have a hearing value in the rang
Possibly the SPS oil-burning Challenger were setup to more-or-less match those on the NP. The NP rosebud coal was said to have a hearing value in the range 6200 to 8000 BTU/lb ex mine, 10 000 BTU/lb when dried. (Was it dried before use?)
A different view was expressed by LeMassena in his book ‘Articulated Steam Locomotives of North America’. Therein he said: ‘It is little recognized that these SP&S articulateds were among the most powerful steam locomotives ever constructed, and they were able to deliver more power than those of the NP because the latter burned a low grade of coal instead of oil.’ One supposes that LeMassena had quantitative evidence to support that statement, and did not simply infer it from the relative fuel heating values.
An interesting case was the WP 2-8-8-2, built as an oil-burner, but said to have a notional grate area of 145 ft². The firebox was 204⅛ inches long x 102¼ inches wide. The same design was used as the basis for the DM&IR 2-8-8-4, which burned coal with a heating value of 13 500 BTU/lb. Here the firebox was slightly longer, at 210⅛ inches, but the same width. But the grate area, presumably chosen to suit the coal used, but also to allow the use of a Gaines wall, was 125 ft²; it did not occupy the whole firebox length.
Cheers,
Kratville, in his book on the UP Challengers, said the following in respect of the six “diverted” to the DRGW:
‘The Union Pacific applied in late 1942 for the 3975-3999 series, the engines being finally delivered in mid-1943. The road desired more units but that is the most the W.P.B. calculated could be put through Alco in the light orders for other roads.
‘At the same time, the W.P.B. was assisting the Denver and Rio Grande Western gear up for the eventual Pacific Theatre operations transportation shift and added six additional Challengers to the U.P. order. The D.&R.G.W. made it clear that it probably would not want the six units after the war particularly because of having to keep parts in stock for just six locomotives. The Union Pacific did not offer to take them either since by this time Jabelmann was already laying out his own plans to dieselize the road right after the war.’
Thus, it looks as if it was predetermined that the DRGW would not keep these locomotives beyond the end of WWII.
Cheers,
Returning to the DRGW six Alco Challengers, ‘Train Shed Cyclopedia (TSC) #45’ has a tabulation of steam locomotive orders and deliveries from 1939 onwards. It shows that DRGW placed an order for six 4-6-6-4s from Baldwin in 1942 May, but that this was not approved by the WPB. An alternative order for six Alco 4-6-6-4s was shown as placed in 1942 June, and approved by the WPB. The UP order for 25 was shown as placed in 1942 February and approved by the WPB.
The UP “big” Challenger was a substantially new design, but it had slipped in before the WPB constraints were applied. TSC #45 shows that the initial order for 20 was placed in 1941 May for 1942 delivery.
If one wanted to make a broad classification of the Challengers by basic design and builder, then the following, with arbitrary numbering, might work:
Alco type 1A 1936 UP, WP
Alco type 1B 1940 D&H, Clinchfield
Alco type 2A 1936 NP, SPS (earlier)
Alco type 2B 1941 NP, SPS (later)
Alco type 3 1942 UP, DRGW
Baldwin type 1 1938 DRGW
Baldwin type 2 1940 WM
Some of the Challengers were described in ‘Railway Age’ and ‘Railway Mechanical Engineer’ articles at the time of their appearance. Known such articles are:
UP ‘Small’ RA 1936 December19 pp.900-903
UP ‘Small’ RME 1937 January pp.1-7
NP RA 1937 March 06 pp.389-391
NP RME 1937 April pp.160-163
DRGW RA 1938 July 09 pp.42-44,70
DRGW RME 1938 September pp.323-329
D&H RA 1940 August 10 pp.207-218
D&H RME 1940 September pp.337-344
WM RA 1941 January 25 pp.209-215
WM RME 1941 February pp.45-52
UP ‘Big’ RA 1942 October 03 pp.516-519
UP ‘Big’ RME 1942 October pp.413-417
As I think is well-known, both journals are available at the Internet Archive, RME under its later name of ‘Railway Locomotives and Cars’.
https://archive.org/details/pub_railway-age?sort=-date&and[]=year%3A"1937"
https://archive.org/details/pub_railway-locomotives-and-cars
Cheers,
Bringing this thread back a little bit, does anyone have the unit costs of these? I’ve seen some numbers thrown around for NP Z-6’s (185,000 ish) the UP CSA-1’s (130,000 ish) and the last UP Challengers (225,000 ish) but apart from the Z-6 I dont really have faith in these numbers. Would be interesting to see how costs stack up.
I thought this was an interesting set of questions and wanted to revisit it to see if anyone with more knowledge on the subject
There are a number of references, none yet quite rigorous, which point out the superiority of a proper deep firebox with circulators over something with a wide grate ‘over the drivers’. This is further enhanced if active circulation is provided in the water legs (a la Cunningham circulator, which draws from a downcoming region in the convection section of the boiler, and uses a jet pump to distribute it through nozzles in the outer wrapper above the mud ring).
The deep firebox implies greater mass on the trailing truck, both from the additional metal structure and weight of water. Note that on C&O, which had dramatically high axle-load capacity, the Alleghenies have a six-wheel trailing truck.
Rear-end stability on a deep-firebox engine of suitable capacity is almost incomparably better than a Challenger. Look at the Bissel formula that keeps the truck wheelbase ‘normal to the railhead’ in curves, then extend the truck out so rear bearing and steering forces are as far outboard and to the rear of the chassis as possible, and angle the restoring-force devices (usually rockers or segments of gears) to match the swing radius at the rear.
In a pinch, you could use the dodge that was introduced in the ‘intermediate’ Berkshire trailing truck frame, when a long frame pivoted at the original ‘articulation’ point was used as a Delta-style trailer. This was treated dynamically as a long 2-wheel Delta trailer, with the leading axle only weight-bearing – it could float laterally on a pair of hardened-steel rollers independently of truck-frame angularity. Any of the subsequent schemes of lateral-motion compliance could be used on such an axle if desired.
The only American engines with a deep firebox and divided drive that used a four-wheel lead truck were the PRR Qs, and those did not have the ‘compound-pendulum’ guiding concerns of a Mallet-style chassis. All the six- and eight-coupled simple articulateds with
Forgive my ignorance but would a deep firebox as described be just as useful when using the lesser quality coal utilized by many of the roads that employed Challengers? I was under the impression that the shallow firebox was necessary to increase volume to make up for the lower-energy coal being used, and as such were a necessary compromise.
For the sake of discussion how would a N&W A run using UP Sub-Bituminous coal, or in an even more extreme case Northern Pacific Rosebud coal? Conversely, how would a wide, shallow firebox Challenger run using the high BTU coal used on the N&W?
Keep in mind that the effective tube/flue length in any modern boiler is in the range of 20 - 24’. Anything longer than that is a waste, even if used in a Chapelon or Porta sectional arrangement.
The issue with burning subbituminous ‘correctly’ is that the fuel is extremely friable, and breaks up and levitates in the combustion plume. You will see ignorant railfans saying something like “40% of the coal never hits the grates” and indeed it doesn’t, but it burns similar to carbureted oil, with some of the same luminous-flame advantage that oil has.
In order to get this effect with a typical ‘deep’ firebox, you would need a method of firing either capable of placing fuel far down under the arch, or feeding at the throat as many methods of oil firing do to get the longest plume. In practice that would be more difficult than the methods used on Challengers.
There are a couple of presumptions: that the lavish amount of fuel to produce equivalent mass flow of steam is available, and that the net cost of all the fuel is lower than bituminous or oil. A more important consideration (since 1970, nearly a decade after it ceased to matter on UP) is the extraordinarily dirty exhaust from that method of firing using nothing more complicated than a regular stoker arrangement for control. Careful tinkering with the secondary-air arrangements and providing effective combustion-air preheat might go a long way toward addressing this stuff.
You wouldn’t fire an A on subbituminous or any other low-rank coal; it wasn’t designed for that. It might be said that the Allegheny could be run on lower-quality coal – but I suspect it, too, would not do well. That might be different if you arranged to feed the fuel from the throat, something that was the subject of a great deal of development in the late Thirties (and that, as discussed in a couple of other threads, might have been one of the great epic failures of steam tech
Moving from a component analysis to an overall viewpoint, the locomotive needs to be able to do the intended haulage job with acceptable reliability and maintenance costs, burn the available/preferred fuel, be an acceptable vehicle on the track throughout its speed range, and to the extent reasonable, follow the particular railroad’s established patterns and preferences.
In particular, the firebox/boiler and running gear must fit each other. That requirement could entail that one or both depart, a little or perhaps a lot, from particularist ideals as to how they should look. Detail design then becomes a series of tradesoff. In some situations, it is likely that more than one design approach would do the required job at acceptable cost.
In the case of the x-6-6-y articulated, there was a tradeoff to be made between the pilot truck and firebox configurations. If a four-wheel pilot truck was desired to keep lateral railhead forces within the designated system boundary, then a long, shallow firebox was dictated. If a deep firebox was desired for the best combustion of good bituminous coal, then a two-wheel pilot truck was dictated. Neither tradeoff forced a “fatal” component choice. Also, it is not clear that the deep firebox was necessarily better when it came to burning lower quality sub-bituminous coal, or oil. With the latter, firebox length appears to be a significant parameter, in which case the Challenger firebox might have been well-matched to the task.
Duffy, in his paper “Technomorphology and the Stephenson Traction System” (1), made some interesting observations on the components and systems viewpoint, as follows.
‘Hence, a transformed component must be related to the larger system if its significance is to be gauged, because assessment of a particular technology can change if the view embraces the whole system instead of being limited to the component. The ‘scale’ of the view taken—whether ‘components scale’ or ‘systems scale’—decided policy towards steam traction in the 1940s and 1950s.
‘Those engineers who took the ‘systems’ view (which embraced the whole railway with its role in the economy and general technology) recognised the impending obsolescence of steam power in time to introduce superior traction modes in an economical manner. Those who failed to take the broad view, such as Chapelon and Riddles, wasted valuable resources and time in developing steam locomotives when they were obsolete. There were also shifts in the scale of the view taken within the steam traction machine-ensemble. Engineers such as Chapelon and Lawford Fry, tended to design high-powered, high thermal efficiency, often complex types. The more modern minded, who took the larger scale view embracing the entire machine-ensemble, simplified the locomotive and reformed the system through better management.
‘Some belonged to the Churchward, Gresley, Bulleid tradition, believing that it would be through ever-increasing ingenuity in design that the locomotive would rise to increasing heights of performance. Others were inclined to the Stanier, Thompson, Riddles opinion that it would be through simplification in design and good organisation for maintenance that progress would be made.’
Against that, it seems reasonable that locomotive design details such as firebox shape may have had less significance in the whole system, that is with the locomotive as a part in the whole railroad system, than when looked at individually.
Back in the 1990s, there was a magazine article (Trains or RF&RR – I don’t recall which) on UP 3985 either by Steve Lee (of the then UP steam team) or including commentary from him. There was some comparison with the N&W 2-6-6-4; in respect of the firebox shape, Steve Lee’s viewpoint was that in practice, it didn’t matter. He might not have been without bias, but on the other hand, in big picture system terms, he was probably right.
In respect of the UP, I understand that Jabelman would have acquired diesels has they been available rather than the additional Challengers, Big Boys and FEFs in 1944. Although he was a steam locomotive designer, he evidently took the broad view that modern traction was a much bigger step forward in terms of the whole railroad system than would be any practicable and significant improvement in the steam locomotive.
Regarding two-wheel versus four-wheel pilot trucks, it is evident that to a first approximation, for the same lateral guiding force the latter would exert half the lateral railhead force. (Accepting that variations in moment arm lengths, etc., would introduce some variation away from the 1:2 ratio.) As the lateral railhead forces increase with the square of the speed, the four-wheel truck would allow 1.4 times the speed of a two-wheel truck for the same lateral railhead force.
One comparative datapoint comes from the (rather pointless) British Railways standard steam locomotive programme. There it was determined that any locomotives that operated habitually above 60 mile/h would be fitted with four-wheel pilot trucks.
Some late four-wheel pilot trucks had variable lateral resistance. I don’t know the numbers for the UP “big” Challengers, but they were probably similar to those for the Big Boy, namely 18% initially, increasing to 33%. Alco seemed to use this approach in conjunction with its lever principle in which all but the trailing set of drivers had lateral motion devices. I don’t know if progressive lateral control was also applied to two-wheel pilot trucks, but if so, I have not seen mention of it. I think that the N&W A had a fixed 30%, but that needs to be confirmed. A possible factor is that unlike a four-wheel truck, a two-wheel pilot truck is not self-stable. Being pushed, as it were, it would be inclined to swing one way or the other, so that the lateral controls would need to be strong enough to keep it steady on tangent track.
As said, turning an articulated locomotive front unit into a curve requires less lateral force than turning a rigid locomotive, but the front unit has the job of acting as a pilot for the remainder of the locomotive. Also, the front units were generally viewed as being potentially problematical in respect of lateral stability, so adequate pilot truck guiding force on tangent track was required.
The European approach of having a two-wheel pilot truck linked to the leading driving axle in order to provide the latter with some lateral motion probably offered some improvement, but it would seem that the use of a lateral motion device on the leading driving axle was a better solution, in that the latter could then adjust itself independently, allowing optimum distribution of lateral railhead forces.
I was not aware that trailing truck stability was a major issue, except maybe for the Lima articulated type as used on the A-1 & co. It would appear that it needed enough lateral resistance to keep it stable on tangent track, but not so much that it undid the good work done by the pilot truck in curving. That seemed to mean that it had about half the lateral resistance of the pilot truck. The Big Boy had 10% initial, increasing to 18%. (Trailing trucks competing with pilot trucks was a problem with bidirectional rigid-frame electric locomotives, such as the 1-D-1 and 2-D-2 types, but that is a separate issue.)
An interesting aspect of the N&W A was that it had a single-plane articulated joint. This, when it had relatively limited clearances that severely restricted pitch axis motion, was held to offer a significant improvement in front unit stability. N&W appears to have been the first to use it, although it did not call it out as a significant feature, nor, as best I can ascertain, did it patent it. Alco had addressed the problem differently in the case of the original UP Challenger. The conventional two-plane articulation joint was friction damped on its pitch axis. This though caused undue stress on the built-up front unit frame. Alco first used the single-plane joint on the D&H Challenger in 1940. Lima used it on the C&O Allegheny; Baldwin’s first use was on the B&O 2-8-8-4. A perhaps surprising application was on the final Baldwin 2-6-6-2 batch for the C&O. Way back when (late 1980s, I think), I was looking at the example in the Baltimore museum, and did a double take when I saw the articulation joint. And this was with built-up, not cast frames.
(1) M.C, Duffy (1982) Technomorphology and the Stephenson Traction System, Transactions of the Newcomen Society.
Cheers,
Western Maryland evidently made dynamometer car measurements on some of its steam locomotives, including the M-2 class Challenger, for which the drawbar power curve showed a peak of roundly 4700 hp at 50 mile/h, having reached 4250 hp at 30 mile/h. The precision, repeatability and reproducibility of the curve and this number is unknown. (Although back then, I doubt that latter-day ASTM-style precision and bias statements in respect of test methods and data were very common at all.)
The UP provided calculated curves. For the “small” Challenger, these showed a drawbar power peak of about 4550 hp at 33 mile/h, declining to 4400 hp at 50 mile/h. For the “big” Challenger, the numbers were 4750 hp at 34 mile/h, declining to 4450 hp at 50 mile/h. How these calculated curves compared with the actuals is unknown.
I suspect that it would be unjustified to interpret these numbers with anything tighter than around a ±10% tolerance range. In that light, we could say that all three were of broadly similar power output, with the UP “big” design being directionally more powerful than the “small”, and the WM design, closer overall to the UP “big”, having the peak of its power curve somewhat higher in the speed range than the other two. Perhaps the last-mentioned gave rise to the “higher powered than the others” viewpoint?
Although it is the peak power numbers that create “bragging rights”, it probably requires some integration and weighting to derive the relative utilities of a set of power curves. But to put that in perspective, in general, steam locomotive power curves were poorly matched to actual train haulage requirements - i.e. they had much lower utility - as compared with those of modern traction. That was shown quite clearly in P.W. Kiefer’s 1947 book “A Practical Evaluation of Motive Power”. Perhaps one could say that attempting to rank the relative utility of steam locomotive power curves was itself a low-utility exercise.
Cheers,