The "Largest" Steam Locomotives

Page Contents: [The "Largest" Non-Articulated Steam Locomotives] [The "Largest" Articulated Steam Locomotives]

What was the largest steam locomotive?

Often, discussions arise about which steam locomotive was the largest, or which steam locomotive was the most powerful. These questions are difficult to answer without some qualifications. For example, my response to the above question would be:

What do you mean? Longest? Heaviest? Most wheels? Most HP? In the United States only?

You see, this is a hard question to answer (as it is written). When comparing steam locomotives, you really need to be specific as to what you're looking for. For example, an easier question to answer would be: "What was the heaviest, non-articulated steam locomotive in the US?". However, another facter that makes answering this question difficult is that there was generally no standard for these measurements. For example, the weight of a steam locomotive could be taken with the boiler full of water or empty or somewhere in between. An inch of change in the water glass could correspond to several thousand pounds difference in weight. As a result, don't try to take these numbers too literaly.

These tables should answer some of these questions by category.

Non-Articulated Steam Locomotives

Most Pulling Force (Non-Articulated)
RoadClassWheel ArrangementTractive Effort
AT&SFMadam Queen2-10-4113,087
B&LEH1A-G2-10-4102,106
CB&QM-42-10-4102,106
PRRI1sa2-10-0102,027
PRRQ-24-4-6-4100,800 (115,800 w/booster)
UP9000-874-12-296,646
WMI22-10-096,300
C&OT-12-10-495,106
PRRJ-12-10-495,106 (110,100 w/booster)
KCS900-92-10-493,302
CGWT1/T2/T32-10-492,590
T&PI12-10-492,590
D&H14034-8-091,500 (simple) (108,000 w/booster) (four-cylinder, triple-expansion, built 1933 by ALCO. info)
UnionU-108b0-10-290,893
L&NEF-12-10-090,295
CPRT4a 80002-10-490,000 (3 cylinder, dual pressure) info
CB&QM-42-10-490,000 before rebuilding, 83,300 after
B&OS1a2-10-284,300
BLW600004-10-282,500
PRRQ-14-6-4-481,793 (93,043 w/booster)
N&WJ4-8-480,000 (sized for 72,000)
AT&SF2900-294-8-479,968
SPGS-44-8-478,500 w/booster
GNO-82-8-278,000
Note: Pulling force or tractive effort was one of the only quantities that was determined almost entirely from the locomotive's geometry. It was generally the limiting facter in determining the size of the train the locomotive could pull.

Most Powerful (Non-Articulated) *
RoadClassWheel ArrangementHorsepower
Draw Bar (DBHP)Indicated (IHP)
PRRS-16-4-4-67200 (1200 ton train @ 100MPH)
PRRQ-24-4-6-47,987@57.4 (Altoona test plant)
PRRS-26-8-66,900
PRRT14-4-4-46,110@85.5MPH6,552@85.5MPH
SPGS-44-8-45,500@55MPH
N&WJ4-8-45,300@40MPH6,000
NYCS-1-b4-8-45,070@85mph (w/o tender)6,680
AT&SFMadam Queen2-10-45,000
UP 4-12-24,750
* Note: Stating the horsepower of a steam locomotive is a difficult thing to do. The horsepower had to be measured either at the cylinder or using a dynamometer car and would change depending on the speed of the locomotive. Even if the horsepower was measured, the power depends upon many things including the quality of the fuel and how well the locomotive was being fired. As a result, two identical locomotives tested on different days operated by different engineers may yield different results. Even though some of these figures come from books, many of them are argued to be unreliable. Also, the specificity of some of these numbers would seem to suggest very accurate quantities when round figures would be probably safer to use. As a result, these figures should be taken with a grain of salt.
DBHP: Drawbar Horsepower
IHP: Indicated Horsepower at cylinders
CHP: Calculated Horsepower

Longest (Non-Articulated)
RoadClassWheel ArrangementEngine + Tender = Total
PRRS-16-4-4-677'-9" + 62'-3" = 140'-0"
PRRQ-24-4-6-4124'-7"
PRRQ-14-6-4-4122'-10"
PRRS-26-8-6122'-7"
AT&SF 2-10-466'-3" + 55'-0" = 121'-3"
AT&SF29004-8-464'-5" + 55'-6" = 119'-11"
PRRT14-4-4-468'-2" + 51'-7" = 119'-9"
PRRJ-12-10-4117'-8"
NYCS14-8-463'-5" + 52'-0" = 115'-5"
AT&SFMadam Queen2-10-4111'-11 1/4"
UP 4-12-264'-0" + 37'-8" = 101'-8"
Note: One would think that the length of a steam locomotive would be an easy quantity to provide accurately. However, even if the length was taken from coupler knuckle to couple knuckle, the play in the draft gear can account for several inches of difference.

Heaviest (Non-Articulated) *
RoadClassWheel ArrangementOn DriversEngineTender¤Total
PRRQ-24-4-6-4 619,100434,0001,053,100
PRRS-16-4-4-6280,000608,170451,8301,060,000
PRRQ-14-6-4-4 593,500434,3701,027,870
PRRS-26-8-6 589,920442,1801,032,100
PRRJ-12-10-4 575,800411,580987,380
AT&SFMadam Queen2-10-4   877,600
B&LEH1A-G2-10-4353,000524,000  
UP9063-874-12-2 515,000277,000792,000
AT&SF29004-8-4 510,700450,300961,000
SP&SE14-8-4296,000   
PRRT14-4-4-4 491,020439,180930,200
* NOTE: The true weight of a steam locomotive is difficult to know. The weight of a locomotive was usually measured with a boiler "filled" with water or with the tender filled with coal/oil and water. However, a difference of 1/2 inch of water in the water glass of a large steam locomotive could result in several thousand pounds difference in weight. Likewise, the tender weight was very variable. As a result, the figures listed should be considered a rough estimate.

Articulated Steam Locomotives

Most Pulling Force (Articulated)
RoadClassWheel ArrangementTractive Effort
N&WJawn HenryC+C+C+C180,000
VirginianX-A2-8-8-8-4166,300 (compound) 199,560 (simple)
VirginianAE2-10-10-2147,200 (compound) 176,600 (simple)
N&WY6b2-8-8-2170,000 (simple expansion mode, with booster)
PennsylvaniaHC1s2-8-8-0167,325
GNR-22-8-8-2162,475
ErieP-12-8-8-8-2160,000
NPZ-52-8-8-4159,330 (145,930 + 13,400 booster)
GNR-22-8-8-2153,000
N&WY6b2-8-8-2152,206 (simple expansion mode, before mid-1950 modifications)
WPM-137/1512-8-8-2151,000 (with Franklin trailing truck booster) 137,000 (without booster)
NPZ-52-8-8-4145,930
GNR-1s2-8-8-2142,165
D&RGWL-1312-8-8-2140,093
DM&IRM-42-8-8-4140,093
WPM-1372-8-8-2137,174
UP 4-8-8-4135,375
Note: Pulling force or tractive effort was one of the only quantities that was determined almost entirely from the locomotive's geometry. It was generally the limiting facter in determining the size of the train the locomotive could pull.

Most Powerful (Articulated) *
RoadClassWheel ArrangementHorsepower
Draw Bar (DBHP)Indicated (IHP)
C&OH-82-6-6-67,498@46MPH
WMM-24-6-6-46,345@50MPH
UP 4-8-8-46,298@41.5MPH6,000@37MPH (CHP)
DM&IRM-42-8-8-46,250
SPAC-124-8-8-26,000@40MPH
N&WY6b2-8-8-25,600@25MPH (simple expansion mode, with booster)
N&WA2-6-6-45,300@43MPH6,800@38MPH
C&OM-14-8-0-4-8-43,0006,000 (turbine)
* Note: Stating the horsepower of a steam locomotive is a difficult thing to do. The horsepower had to be measured either at the cylinder or using a dynamometer car and would change depending on the speed of the locomotive. Even if the horsepower was measured, the power depends upon many things including the quality of the fuel and how well the locomotive was being fired. As a result, two identical locomotives tested on different days operated by different engineers may yield different results. Even though some of these figures come from books, many of them are argued to be unreliable. Also, the specificity of some of these numbers would seem to suggest very accurate quantities when round figures would be probably safer to use. As a result, these figures should be taken with a grain of salt.
DBHP: Drawbar Horsepower
IHP: Indicated Horsepower at cylinders
CHP: Calculated Horsepower

Longest (Articulated)
RoadClassWheel ArrangementEngine + Tender = Total
N&WJawn HenryC+C+C+C111'-7" + 50'-0" = 161'-1"
C&OM-14-8-0-4-8-4106'-0" + 48'-0" = 154'-0"
UP 4-8-8-485'-10" + 47'-0" = 132'-10"
DM&IRM-3/M-42-8-8-479'-5" + 47'-5" = 126'-10"
SPAC-92-8-8-479'-0 1/2" + 46'-7 5/8" = 125'-8 1/8"
B&OEM-12-8-8-475'-6 1/2" + 49'-8 1/2" = 125'-3"
NPZ-52-8-8-480'-7 5/8" (80'-5 5/8") + 44'-6" = 125'-1 5/8" (124'-11 5/8")
C&OH-82-6-6-676'-8" + 47'-8" = 124'-4"
SPAC-124-8-8-278'-11" + 44'-10" = 123'-8"
Note: One would think that the length of a steam locomotive would be an easy quantity to provide accurately. However, even if the length was taken from coupler knuckle to couple knuckle, the play in the draft gear and buffer/chafing plate can account for up to a foot of difference.

Heaviest (Articulated) (based on engine weight) *
RoadClassWheel ArrangementOn DriversEngineTender¤Total
C&OM-14-8-0-4-8-4 856,000377,9701,233,970
ErieP-12-8-8-8-2761,600853,050316,7001,169,750
VirginianTriplex2-8-8-8-4725,475842,310  
N&WJawn HenryC+C+C+C 818,000354,0001,172,000
UP24-8-8-4545,200772,250436,5001,208,750
C&OH-82-6-6-6471,000778,000 (stated in books)
775,330 (Lima's re-weighing)
320,5401,098,540
VirginianAG2-6-6-6495,000753,000442,0001,195,000
NPZ-52-8-8-4553,000717,000408,4001,125,400
DM&IRM-42-8-8-4564,974699,700438,3351,138,035
SPAC-92-8-8-4522,200677,200320,800998,000
WPM-137/1512-8-8-2552,700665,100408,2501,073,350
SPAC-8,10,11,124-8-8-2531,700657,900393,3001,051,200
D&RGWL-1312-8-8-2559,500649,000343,000992,000
NP/SP&SZ-84-6-6-4444,000644,000437,0001,081,000
D&RGWL-1054-6-6-4437,939641,900394,0001,035,900
D&RGWL-622-6-6-2295,500640,000159,000799,000
UP4664-54-6-6-4404,200634,500434,5001,069,000
B&OEM-12-8-8-4485,000628,700382,0001,010,700
* NOTE: The true weight of a steam locomotive is difficult to know. The weight of a locomotive was usually measured with a boiler "filled" with water. However, a difference of 1/2 inch of water in the water glass of a large steam locomotive could result in several thousand pounds difference in weight. As a result, the figures listed should be considered a rough estimate.
¤ NOTE: Tender weights are for empty tenders.

About Horsepower

Comparing the horsepower of various steam locomotives can be a difficult thing to do. One of the reasons for this is that on a steam locomotive there are several ways to measure horsepower (drawbar horsepower, indicated or cylinder horsepower, and calculated horsepower). Also, drawbar horsepower is measured with a dynamometer car where the firing rate and track gradient may effect the results. Often, books do not specify which figures they are quoting. It is also important to know that the horsepower rating of steam locomotives just wasn't as important as their tractive effort rating. The reason for this is because for most steam locomotives, the limiting factor was how much tonnage a locomotive could move from a standing start (tractive effort). In general, if a steam locomotive can get a train rolling, it could pull it at an efficient speed. For diesels, the opposite is true. Most any diesel can generate enough tractive effort to get any train rolling, however, a smaller diesel engine may not be able to pull it very fast. I've seen this said on the net in a very clever way: "A steam locomotive of the right size will run the wheels off a train it can't possibly get started. A diesel-electric set can start a train it can't run." This article is a well-written comparison of steam versus diesel.

The N&W Y6b

In 1952, the Norfolk & Western ran several tests. These tests compared a four- unit F7 consist against a modified class A (2-6-6-4 number 1239) and later a modified class Y6b (2-8-8-2 number 2197). Several modifications were made to 2197 which made it different from the other Y6b locomotives. These modifications included a "booster valve", a new "intercepting/reducing valve" which increased its drawbar horsepower by 26% and its drawbar tractive effort by 15%. An article in the November 1991 issue of TRAINS titled N&W's Secret Weapons goes into more detail about these tests. The author of this article nicknamed the modified 2197 a "Y6c".

In the mid-1950s, when the older 2100 series locomotives were "shopped", they were fitted with these same modifications. According to the author, these retro-fitted "Ys" could now produce 170,000 pounds of tractive effort and 5,600 drawbar horsepower. Many consider these numbers to be erroneous.

But I Thought the Big Boy was the Largest!

You may have noticed that UPs Big Boys did not "win" any of the above categories (except that without the tender, it has the largest engine body of all reciprocating steam locomotives). Many books will cite that the Big Boy is the "largest" steam locomotive ever built. Why is this?

Even though the Big Boys did not "win" any of the categories listed above, they always "placed". If you were to eliminate all of the unsuccessful and test locomotives from the above tables and then look at the overall ratings of the remaining steam locomotives in the above categories, it would become clear that the Big Boys (along with the N&W Y6 and A, DM&IR M, and perhaps the C&O H-8 locomotives) were the "largest" among all successful steam locomotives.

For those interested in discussions about what locomotive was the largest, there is an interesting archive of information here.

What is Tractive Effort?

Tractive effort was a theoretical quantity. Railroads preferred it to HP ratings because HP involved a time quantity which was determined, in part, by how well the locomotive was being fired (among many other variables). Tractive effort, on the other hand, was determined strictly by the geometry of the locomotive. Tractive effort can be determined by the following equation:
          c P (d)^2 s
     TE = -----------
               D
TE = tractive effort in lbs
c = a constant determined by the mean effective pressure and friction (usually 85%)
P = boiler pressure
d = piston diameter
s = piston stroke
D = driver diameter

Using the above equation with specifications for a Big Boy yields:
Boiler pressure: 300 lbs
Cylinders: 2x 23.75x32 inches
Drivers: 68 inch

          .85 300 2(23.75)^2 32
     TE = --------------------- = 135,375 lbs
                   68

Try designing your own steam locomotive.

Why is the Weight on Drivers Important?

For each revolution of a two-cylinder steam locomotive drive wheel, torque is applied four times (or at a maximum at four different times per complete rotation). As a result, it is easy for the drive wheels to slip. To keep the steam locomotive from being too "slippery", it is important to have sufficient weight on the drive wheels. For a given steam locomotive, the ratio of the weight on drivers divided by the tractive effort is called the factor of adhesion. It has been found that a factor of adhesion of around 4 is a good balance of pulling force and engine weight. If the factor of adhesion is too low (3.5, for example), the locomotive will be "slippery".