The "Largest" Steam Locomotives

What was the largest steam locomotive?

Often, discussions arise about which steam locomotive was the largest, biggest, or strongest. These questions are difficult to answer without being more specific. For example, my response to the above question would be:

What do you mean? How do you define "largest"? 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 more 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.

This page will try to answer some of these types of questions.

Non-Articulated Steam Locomotives

Most Pulling Force (Non-Articulated) About Tractive Effort

Tractive Effort

Tractive Effort is a measure of the force that a locomotive can apply to the train it is pulling. For steam locomotives, Tractive Effort was usually the limitting factor in the size of the train it could pull. Therefore, Tractive Effort was more important than HorsePower for steam locomotives.

Tractive Effort is a quantity determined almost entirely from the locomotive's geometry. It is at a maximum when the locomotive is motionless and is therefore sometimes called Starting Tractive Effort. As the locomotive gains speed, Tractive Effort decreases.

The equation for Tractive Effort is

          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

Road	Class	Wheel Arrangement	Number Built	Year Built	Tractive Effort
AT&SF	Madam Queen	2-10-4	1 + 35	1930, 1938	113,087
B&LE	H1A-G	2-10-4	47	1929	102,106
CB&Q	M-4	2-10-4	18	1927	102,106
PRR	I1sa	2-10-0	973	1916-1929	102,027
PRR	Q-2	4-4-6-4	26	1945	100,800 (115,800 w/booster; Pennsy Power, Alvin Staufer)
UP	9000-87	4-12-2	88	1926-1930	96,646
WM	I2	2-10-0	20	1927	96,300
C&O	T-1	2-10-4	40	1930	95,106
PRR	J-1	2-10-4	125	1942	95,106 (110,100 w/booster)
KCS	900-9	2-10-4	10	1937	93,302
CGW	T1/T2/T3	2-10-4	36	1929	92,590
T&P	I1	2-10-4	70	1925	92,590
D&H	E-7 (1403)	4-8-0	1	1933	91,500 (simple) (108,000 w/booster) (four-cylinder, triple-expansion, built 1933 by ALCO. info)
Union	U-108b	0-10-2	9	1936	90,893
L&NE	F-1	2-10-0	4	1927-1931	90,295
CPR	8000	2-10-4	1	1931	90,000 (3 cylinder, dual pressure)
CB&Q	M-4	2-10-4	18	1927	90,000 before rebuilding, 83,300 after
B&O	S1a	2-10-2	125	1923-1926	84,300
Baldwin	60000	4-10-2	1	1926	82,500
PRR	Q-1	4-6-4-4	1	1944	81,793 (93,043 w/booster; Pennsy Power, Alvin Staufer)
N&W	J	4-8-4	14	1943	80,000 (sized for 72,000)
AT&SF	2900	4-8-4	30	1944	79,968
SP	GS-4/5	4-8-4	30	1941,42	78,500 w/booster
GN	O-8	2-8-2	20	1932	78,000

Most Powerful (Non-Articulated) About HP

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

Road	Class	Wheel Arrangement	Number Built	Year Built	Horsepower
Road	Class	Wheel Arrangement	Number Built	Year Built	Draw Bar (DBHP)	Indicated (IHP)
PRR	S-1	6-4-4-6	1	1939	7200 (1200 ton train @ 100MPH)
PRR	Q-2	4-4-6-4	26	1945		7,987@57.4 mph (Altoona test plant; Pennsy Power, Alvin Staufer)
PRR	S-2	6-8-6	1	1944		6,900 HP turbine (Pennsy Power, Alvin Staufer)
PRR	T1	4-4-4-4	50	1945-46	6,110@85.5 mph (Pennsy Power, Alvin Staufer)	6,552@85.5 mph; 4,100@100 mph (Pennsy Power, Alvin Staufer)
SP	GS-4/5	4-8-4	30	1941,42	5,500@55MPH
N&W	J	4-8-4	14	1943	5,300@40MPH	6,000
NYC	S-1b	4-8-4	24	1945	5,070@85mph (w/o tender)	6,680
AT&SF	Madam Queen	2-10-4	1 + 35	1930, 1938	5,000
UP	9000-87	4-12-2	88	1926-1930		4,750

Longest (Non-Articulated) About Length

One would think that the length of a steam locomotive would be an easy quantity to provide and compare. However, diagrams are not consistent in measuring length from the same locations for all locomotives. Wheelbase length is probably more important that overall length and obviously would be a little less than if measuring from the end of the coupler or pilot to the rear frame or coupler of the tender. 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. This chart will show the overall length from coupler or pilot to rear of tender.

Road	Class	Wheel Arrangement	Number Built	Year Built	Engine + Tender = Total	Notes	Source
PRR	S-1	6-4-4-6	1	1939	80'-6" + 59'-8" = 140'-2"
PRR	Q-2	4-4-6-4	26	1945	124'-7"
PRR	Q-1	4-6-4-4	1	1944	122'-10"
PRR	S-2	6-8-6	1	1944	122'-7"
AT&SF	5000-35	2-10-4	36	1930-1944	66'-3" + 55'-0" = 121'-3"	Pilot to tender frame	MR Steam Locomotive Cyclopedia Vol 1
AT&SF	2900	4-8-4	30	1944	64'-5" + 55'-6" = 119'-11"	PIlot to tender frame	MR Steam Locomotive Cyclopedia Vol 1
PRR	T1	4-4-4-4	50	1945-46	68'-2" + 51'-7" = 119'-9"	Coupler to tender frame (6110)	MR Steam Locomotive Cyclopedia Vol 1
PRR	J-1	2-10-4	125	1942	117'-8"
NYC	S-1b	4-8-4	24	1945	63'-5" + 52'-0" = 115'-5"	Coupler to coupler	MR Steam Locomotive Cyclopedia Vol 1
AT&SF	Madam Queen	2-10-4	1 + 35	1930, 1938	111'-11 1/4"
UP	9000-87	4-12-2	88	1926-1930	64'-0" + 37'-8" = 101'-8"	Coupler to coupler	MR Steam Locomotive Cyclopedia Vol 1

Heaviest (Non-Articulated) About Weight

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.

Road	Class	Wheel Arrangement	Number Built	Year Built	On Drivers	Engine	Tender	Total
PRR	Q-2	4-4-6-4	26	1945		619,100	434,000	1,053,100
PRR	S-1	6-4-4-6	1	1939	280,000	608,170	451,830	1,060,000
PRR	Q-1	4-6-4-4	1	1944		593,500	434,370	1,027,870
PRR	S-2	6-8-6	1	1944		589,920	442,180	1,032,100
PRR	J-1	2-10-4	125	1942		575,800	411,580	987,380
AT&SF	Madam Queen	2-10-4	1 + 35	1930, 1938				877,600
B&LE	H1A-G	2-10-4	47	1929	353,000	524,000
UP	9000-87	4-12-2	88	1926-1930		515,000	277,000	792,000
AT&SF	2900	4-8-4	30	1944		510,700	450,300	961,000
SP&S	E1	4-8-4	3	1938	296,000
PRR	T1	4-4-4-4	50	1945-46		491,020	439,180	930,200

Articulated Steam Locomotives

Most Pulling Force (Articulated) About Tractive Effort

Tractive Effort

The equation for Tractive Effort is

          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

Road	Class	Wheel Arrangement	Number Built	Year Built	Tractive Effort
N&W	Jawn Henry	C+C+C+C	1	1954	180,000
Virginian	X-A	2-8-8-8-4	1	1916	166,300 (compound) 199,560 (simple)
Virginian	AE	2-10-10-2	10	1918	147,200 (compound) 176,600 (simple)
N&W	Y6b	2-8-8-2	30	1948-1952	170,000 (simple expansion mode, with booster)
Pennsylvania	HC1s	2-8-8-0	1	1919	167,325
GN	R-2	2-8-8-2	16	1929	162,475
Erie	P-1	2-8-8-8-2	3	1914	160,000
NP	Z-5	2-8-8-4	11	1928	159,330 (145,930 + 13,400 booster)
N&W	Y6b	2-8-8-2	30	1948	152,206 (simple expansion mode, before mid-1950 modifications)
WP	M-137/151	2-8-8-2	10	1931	151,000 (with Franklin trailing truck booster) 137,000 (without booster)
NP	Z-5	2-8-8-4	11	1928	145,930
GN	R-1s	2-8-8-2	10	1927	142,165
D&RGW	L-131/132	2-8-8-2	20	1927-1930	140,093
DM&IR	M-3/4	2-8-8-4	18	1941-1943	140,093
WP	M-137	2-8-8-2	10	1931	137,174
UP		4-8-8-4	25	1941-1944	135,375

Most Powerful (Articulated) About HP

DBHP: Drawbar Horsepower
IHP: Indicated Horsepower at cylinders
CHP: Calculated Horsepower

Road	Class	Wheel Arrangement	Number Built	Year Built	Horsepower
Road	Class	Wheel Arrangement	Number Built	Year Built	Draw Bar (DBHP)	Indicated (IHP)
C&O	H-8	2-6-6-6	60	1941-1948	7,498@46MPH
WM	M-2	4-6-6-4	12	1940-1941	6,345@50MPH
UP		4-8-8-4	25	1941-1944	6,298@41.5 mph (Big Boy, William Kratville)	6,000@37MPH (CHP)
DM&IR	M-3/4	2-8-8-4	18	1941-1943	6,250
SP	AC-6/12	4-8-8-2	169	1930-1944	6,000@40 mph (Those Amazing Cab Forwards, George Harlan, p67)
N&W	Y6b	2-8-8-2	30	1948-1952	5,600@25MPH (simple expansion mode, with booster)
N&W	A	2-6-6-4	43	1936-1950	5,300@43MPH	6,800@38MPH
C&O	M-1	4-8-0-4-8-4	3	1947-1948	3,000	6,000 hp turbine, Guide to North American Steam Locomotives, George Drury

Longest (Articulated) About Length

Road	Class	Wheel Arrangement	Number Built	Year Built	Engine + Tender = Total	Notes	Source
N&W	Jawn Henry	C+C+C+C	1	1954	111'-7" + 50'-0" = 161'-1"
C&O	M-1	4-8-0-4-8-4	3	1947-1948	106'-0" + 48'-0" = 154'-0"
UP		4-8-8-4	25	1941-1944	85'-10" + 47'-0" = 132'-10"	Pilot to tender frame	MR Steam Locomotive Cyclopedia Vol 1
DM&IR	M-3/4	2-8-8-4	18	1941-1943	79'-5" + 47'-5" = 126'-10"	Pilot to tender coupler (estimated)	MR Steam Locomotive Cyclopedia Vol 1
SP	AC-9	2-8-8-4	12	1939	79'-0 1/2" + 46'-7 5/8" = 125'-8 1/8"
B&O	EM-1	2-8-8-4	30	1944	75'-6 1/2" + 49'-8 1/2" = 125'-3"
NP	Z-5	2-8-8-4	11	1928	80'-7 5/8" (80'-5 5/8") + 44'-6" = 125'-1 5/8" (124'-11 5/8")
C&O	H-8	2-6-6-6	60	1941-1948	76'-8" + 47'-8" = 124'-4"	Coupler to tender frame	MR Steam Locomotive Cyclopedia Vol 1
SP	AC-6/12	4-8-8-2	169	1930-1944	79'-4" + 44'-10" = 124'-2"	Coupler to tender frame (estimated)	MR Steam Locomotive Cyclopedia Vol 1

Heaviest (Articulated) (based on engine weight) About Weight

Road	Class	Wheel Arrangement	Number Built	Year Built	On Drivers	Engine	Tender	Total
C&O	M-1	4-8-0-4-8-4	3	1947-1948		856,000	377,970	1,233,970
Erie	P-1	2-8-8-8-2	3	1914	761,600	853,050	316,700	1,169,750
Virginian	X-A	2-8-8-8-4	1	1916	725,475	842,310
N&W	Jawn Henry	C+C+C+C	1	1954		818,000	354,000	1,172,000
UP	2	4-8-8-4	25	1941-1944	545,200	772,250	436,500	1,208,750
C&O	H-8	2-6-6-6	60	1941-1948	471,000	778,000 (stated in books) 775,330 (Lima's re-weighing)	320,540	1,098,540
Virginian	AG	2-6-6-6	8	1945	495,000	753,000	442,000	1,195,000
NP	Z-5	2-8-8-4	11	1928	553,000	717,000	408,400	1,125,400
DM&IR	M-3/4	2-8-8-4	18	1941-1943	564,974	699,700	438,335	1,138,035
Virginian	AE	2-10-10-2	10	1918	617,000	684,000	214,300	898,300
SP	AC-9	2-8-8-4	12	1939	522,200	677,200	320,800	998,000
WP	M-137/151	2-8-8-2	10	1931	552,700	665,100	408,250	1,073,350
SP	AC-8,10,11,12	4-8-8-2	169	1930-1944	531,700	657,900	393,300	1,051,200
D&RGW	L-131/132	2-8-8-2	20	1927-1930	559,500	649,000	343,000	992,000
NP/SP&S	Z-8	4-6-6-4	20	1943	444,000	644,000	437,000	1,081,000
D&RGW	L-105	4-6-6-4	10	1938	437,939	641,900	394,000	1,035,900
D&RGW	L-62	2-6-6-2	8	1910	295,500	640,000	159,000	799,000
UP	4664-5	4-6-6-4	20	1944	404,200	634,500	434,500	1,069,000
B&O	EM-1	2-8-8-4	30	1944	485,000	628,700	382,000	1,010,700

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. This PDF contains a very detailed description of ways of measuring locomotive pulling force and power.

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.

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

Why is the Weight on Drivers Important?

Factor of Adhesion

For each revolution of a two-cylinder steam locomotive drive wheel, torque is at a maximum four times per revolution. Because of this, slippage can be initiated at any of those four maximums. To keep a steam locomotive from being too "slippery", it is important to have sufficient weight on the driving wheels. For any 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 weight on drivers. If the Factor of Adhesion is too low (3.5, for example), the locomotive will be "slippery" when forced to work hard.

Wes Barris