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 longest, non- articulated steam locomotive in the US?".

These tables should answer some of these questions by category.

Non-Articulated Steam Locomotives

Most Pulling Force (Non-Articulated)
Road	Class	Wheel Arrangement	Tractive Effort
PRR	Q-2	4-4-6-4	100,800 (115,800 w/booster)
T&P	I1	2-10-4	97,900
B&LE	H1A-G	2-10-4	96,700
UP	9000-87	4-12-2	96,650
WM	I2	2-10-0	96,300
PRR	I1sa	2-10-0	96,000
AT&SF	Madam Queen	2-10-4	95,584
PRR	J-1	2-10-4	95,100 (110,100 w/booster)
KCS	900-9	2-10-4	93,300
AT&SF	5001	2-10-4	93,000
D&H	1403	4-8-0	91,500 (simple) (108,000 w/booster) (four-cylinder, triple-expansion, built 1933 by ALCO. info)
CPR	T4a 8000	2-10-4	90,000 (3 cylinder, dual pressure) info
CB&Q	M-4	2-10-4	90,000 before rebuilding, 83,300 after
B&O	S1a	2-10-2	84,300
BLW	60000	4-10-2	82,500
PRR	Q-1	4-6-4-4	81,793 (93,043 w/booster)
N&W	J	4-8-4	80,000 (sized for 72,000)
AT&SF	2900-29	4-8-4	79,968
SP	GS-4	4-8-4	78,500 w/booster
GN	O-8	2-8-2	78,000

Most Powerful (Non-Articulated) *

Road Class Wheel Arrangement Horsepower

PRR Q-2 4-4-6-4 7,987@57.4 (IHP)

PRR S-1 6-4-4-6 7200 (DBHP) (1200 ton train @ 100MPH)

PRR S-2 6-8-6 6,900 (IHP)

NYC S-1-b 4-8-4 6,680 (IHP) 5,070@85mph (DBHP w/o tender)

PRR T1 4-4-4-4 6,110@85.5MPH (DBHP)
6,552@85.5MPH (IHP)

SP GS-4 4-8-4 5,500@55MPH (DBHP)

N&W J 4-8-4 5,300@40MPH (DBHP)
6,000 (IHP)

AT&SF Madam Queen 2-10-4 5,000

UP 4-12-2 4,750 (IHP)

* 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. 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. Even though some of these figures come from books, many of them are argued to be unreliable. As a result, these figures should be taken with a grain of salt.
DBHP: Drawbar Horsepower
IHP: Indicated Horsepower at cylinders
CHP: Calculated Horsepower

Most Powerful (Non-Articulated) *
Road	Class	Wheel Arrangement	Horsepower
PRR	Q-2	4-4-6-4	7,987@57.4 (IHP)
PRR	S-1	6-4-4-6	7200 (DBHP) (1200 ton train @ 100MPH)
PRR	S-2	6-8-6	6,900 (IHP)
NYC	S-1-b	4-8-4	6,680 (IHP) 5,070@85mph (DBHP w/o tender)
PRR	T1	4-4-4-4	6,110@85.5MPH (DBHP) 6,552@85.5MPH (IHP)
SP	GS-4	4-8-4	5,500@55MPH (DBHP)
N&W	J	4-8-4	5,300@40MPH (DBHP) 6,000 (IHP)
AT&SF	Madam Queen	2-10-4	5,000
UP		4-12-2	4,750 (IHP)

Longest (Non-Articulated)

Road Class Wheel Arrangement Engine + Tender = Total

PRR S-1 6-4-4-6 77'-9" + 62'-3" = 140'-0"

PRR Q-2 4-4-6-4 124'-7"

PRR Q-1 4-6-4-4 122'-10"

PRR S-2 6-8-6 122'-7"

AT&SF 2-10-4 66'-3" + 55'-0" = 121'-3"

AT&SF 2900 4-8-4 64'-5" + 55'-6" = 119'-11"

PRR T1 4-4-4-4 68'-2" + 51'-7" = 119'-9"

PRR J-1 2-10-4 117'-8"

NYC S1 4-8-4 63'-5" + 52'-0" = 115'-5"

AT&SF Madam Queen 2-10-4 111'-11 1/4"

UP 4-12-2 64'-0" + 37'-8" = 101'-8"

Longest (Non-Articulated)
Road	Class	Wheel Arrangement	Engine + Tender = Total
PRR	S-1	6-4-4-6	77'-9" + 62'-3" = 140'-0"
PRR	Q-2	4-4-6-4	124'-7"
PRR	Q-1	4-6-4-4	122'-10"
PRR	S-2	6-8-6	122'-7"
AT&SF		2-10-4	66'-3" + 55'-0" = 121'-3"
AT&SF	2900	4-8-4	64'-5" + 55'-6" = 119'-11"
PRR	T1	4-4-4-4	68'-2" + 51'-7" = 119'-9"
PRR	J-1	2-10-4	117'-8"
NYC	S1	4-8-4	63'-5" + 52'-0" = 115'-5"
AT&SF	Madam Queen	2-10-4	111'-11 1/4"
UP		4-12-2	64'-0" + 37'-8" = 101'-8"

Heaviest (Non-Articulated) *

Road Class Wheel Arrangement On Drivers Engine Tender¤ Total

PRR Q-2 4-4-6-4 619,100 434,000 1,053,100

PRR S-1 6-4-4-6 280,000 608,170 451,830 1,060,000

PRR Q-1 4-6-4-4 593,500 434,370 1,027,870

PRR S-2 6-8-6 589,920 442,180 1,032,100

PRR J-1 2-10-4 575,800 411,580 987,380

AT&SF Madam Queen 2-10-4 877,600

B&LE H1A-G 2-10-4 353,000 524,000

UP 9063-87 4-12-2 515,000 277,000 792,000

AT&SF 2900 4-8-4 510,700 450,300 961,000

SP&S E1 4-8-4 296,000

PRR T1 4-4-4-4 491,020 439,180 930,200

* 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.

Heaviest (Non-Articulated) *
Road	Class	Wheel Arrangement	On Drivers	Engine	Tender¤	Total
PRR	Q-2	4-4-6-4		619,100	434,000	1,053,100
PRR	S-1	6-4-4-6	280,000	608,170	451,830	1,060,000
PRR	Q-1	4-6-4-4		593,500	434,370	1,027,870
PRR	S-2	6-8-6		589,920	442,180	1,032,100
PRR	J-1	2-10-4		575,800	411,580	987,380
AT&SF	Madam Queen	2-10-4				877,600
B&LE	H1A-G	2-10-4	353,000	524,000
UP	9063-87	4-12-2		515,000	277,000	792,000
AT&SF	2900	4-8-4		510,700	450,300	961,000
SP&S	E1	4-8-4	296,000
PRR	T1	4-4-4-4		491,020	439,180	930,200

Articulated Steam Locomotives

Most Pulling Force (Articulated)
Road	Class	Wheel Arrangement	Tractive Effort
N&W	Jawn Henry	C+C+C+C	180,000
Virginian	X-A	2-8-8-8-4	166,300 (compound) 199,560 (simple)
Virginian	AE	2-10-10-2	147,200 (compound) 176,600 (simple)
N&W	Y6b	2-8-8-2	170,000 (simple expansion mode, with booster)
Erie	P-1	2-8-8-8-2	160,000
GN	R-2	2-8-8-2	153,000
N&W	Y6b	2-8-8-2	152,206 (simple expansion mode, before mid-1950 modifications)
WP	M-137/151	2-8-8-2	151,000 (with Franklin trailing truck booster) 137,000 (without booster)
DM&IR	M-4	2-8-8-4	140,000
UP		4-8-8-4	135,375

Most Powerful (Articulated) *

Road Class Wheel Arrangement Horsepower

C&O H-8 2-6-6-6 7,498@46MPH

N&W A 2-6-6-4 5,300@43MPH (DBHP)
6,800@38MPH (IHP)

WM M-2 4-6-6-4 6,345@50MPH

DM&IR M-4 2-8-8-4 6,250

SP AC-12 4-8-8-2 6,000@40MPH

UP 4-8-8-4 6,200 (DBHP)
6,000@37MPH (CHP)

C&O M-1 4-8-0-4-8-4 6,000 (turbine)
3,000 (DBHP)

N&W Y6b 2-8-8-2 5,600@25MPH (simple expansion mode, with booster)

* 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. 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. Even though some of these figures come from books, many of them are argued to be unreliable. As a result, these figures should be taken with a grain of salt.
DBHP: Drawbar Horsepower
IHP: Indicated Horsepower at cylinders
CHP: Calculated Horsepower

Most Powerful (Articulated) *
Road	Class	Wheel Arrangement	Horsepower
C&O	H-8	2-6-6-6	7,498@46MPH
N&W	A	2-6-6-4	5,300@43MPH (DBHP) 6,800@38MPH (IHP)
WM	M-2	4-6-6-4	6,345@50MPH
DM&IR	M-4	2-8-8-4	6,250
SP	AC-12	4-8-8-2	6,000@40MPH
UP		4-8-8-4	6,200 (DBHP) 6,000@37MPH (CHP)
C&O	M-1	4-8-0-4-8-4	6,000 (turbine) 3,000 (DBHP)
N&W	Y6b	2-8-8-2	5,600@25MPH (simple expansion mode, with booster)

Longest (Articulated)

Road Class Wheel Arrangement Engine + Tender = Total

N&W Jawn Henry C+C+C+C 111'-7" + 50'-0" = 161'-1"

C&O M-1 4-8-0-4-8-4 106'-0" + 48'-0" = 154'-0"

UP 4-8-8-4 85'-10" + 47'-0" = 132'-10"

B&O EM-1 2-8-8-4 125'-3"

DM&IR M-4 2-8-8-4 77'-5" + 47'-7" = 125'-0"

C&O H-8 2-6-6-6 76'-8" + 47'-8" = 124'-4"

SP AC-12 4-8-8-2 78'-11" + 44'-10" = 123'-8"

Longest (Articulated)
Road	Class	Wheel Arrangement	Engine + Tender = Total
N&W	Jawn Henry	C+C+C+C	111'-7" + 50'-0" = 161'-1"
C&O	M-1	4-8-0-4-8-4	106'-0" + 48'-0" = 154'-0"
UP		4-8-8-4	85'-10" + 47'-0" = 132'-10"
B&O	EM-1	2-8-8-4	125'-3"
DM&IR	M-4	2-8-8-4	77'-5" + 47'-7" = 125'-0"
C&O	H-8	2-6-6-6	76'-8" + 47'-8" = 124'-4"
SP	AC-12	4-8-8-2	78'-11" + 44'-10" = 123'-8"

Heaviest (Articulated) *

Road Class Wheel Arrangement On Drivers Engine Tender¤ Total

C&O M-1 4-8-0-4-8-4 856,000 377,970 1,233,970

N&W Jawn Henry C+C+C+C 818,000 354,000 1,172,000

UP 2 4-8-8-4 545,200 772,250 436,500 1,208,750

C&O H-8 2-6-6-6 471,000 778,000 (stated in books)
775,330 (Lima's re-weighing) 320,540 1,098,540

NP Z-5 2-8-8-4 553,000 717,000 408,400 1,125,400

DM&IR M-4 2-8-8-4 564,974 699,700 438,335 1,138,035

WP M-137/151 2-8-8-2 633,000

SP AC-8,10,11,12 4-8-8-2 531,700 657,900 393,300 1,051,200

B&O EM-1 2-8-8-4 485,000 629,000 382,000 1,011,000

* 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 "fully" loaded tenders.

Heaviest (Articulated) *
Road	Class	Wheel Arrangement	On Drivers	Engine	Tender¤	Total
C&O	M-1	4-8-0-4-8-4		856,000	377,970	1,233,970
N&W	Jawn Henry	C+C+C+C		818,000	354,000	1,172,000
UP	2	4-8-8-4	545,200	772,250	436,500	1,208,750
C&O	H-8	2-6-6-6	471,000	778,000 (stated in books) 775,330 (Lima's re-weighing)	320,540	1,098,540
NP	Z-5	2-8-8-4	553,000	717,000	408,400	1,125,400
DM&IR	M-4	2-8-8-4	564,974	699,700	438,335	1,138,035
WP	M-137/151	2-8-8-2		633,000
SP	AC-8,10,11,12	4-8-8-2	531,700	657,900	393,300	1,051,200
B&O	EM-1	2-8-8-4	485,000	629,000	382,000	1,011,000

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.

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.

Spiffy new Java version.

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".