A duplex locomotive is a steam locomotive that divides the driving
force on its wheels by using two pairs of cylinders rigidly mounted to
a single locomotive frame; it is not an articulated locomotive. The
concept was first used in France in 1863, but was particularly
developed in the early 1930s by the Baldwin Locomotive Works, the
largest commercial builder of steam locomotives in North America,
under the supervision of its then chief engineer, Ralph P. Johnson.
Prior to this, the term duplex locomotive was sometimes applied to
articulated locomotives in general.
1 Drawbacks of the 2-cylinder locomotive
2 The duplex solution
3 Baltimore and Ohio class N-1 #5600 George H. Emerson
4 PRR class S1
5 PRR class T1
6 PRR Q1
7 PRR Q2
8 A successful French duplex
Drawbacks of the 2-cylinder locomotive
While the side rods (UK: coupling rods) of a locomotive can be
completely balanced by weights on the driving wheels since their
motion is completely rotational, the reciprocating motions of the
pistons, piston rods, main rods and valve gear cannot be balanced in
this way. A two-cylinder locomotive has its two cranks "quartered" —
set at 90° apart — so that the four power strokes of the
double-acting pistons are evenly distributed around the cycle and
there are no points at which both cylinders are at top or bottom dead
center simultaneously. A four-cylinder locomotive can be completely
balanced in the longitudinal and vertical axes, although there are
some rocking and twisting motions which can be dealt with in the
locomotive's suspension and centering; a three-cylinder locomotive can
also be better balanced, but a two-cylinder locomotive only balanced
for rotation will surge fore and aft. Additional balance weight —
"overbalance" — can be added to damp this, but at the cost of adding
vertical forces, which are known technically as "dynamic augment" and
colloquially as "hammer blow". This can be extremely damaging to the
track, and in extreme cases can actually cause the driving wheels to
leave the track entirely. The heavier the reciprocating machinery, the
greater these forces are, and the greater a problem this becomes.
The duplex solution
In Europe this problem was often overcome by dividing the drive
between inside and outside cylinders, or else by using Articulated
locomotives, although at the time it was not believed possible to run
one stably at greater than 50 mph (80 km/h). American
railroads proved to be unwilling to use locomotives with inside
cylinders, so the problem of balance could not be solved by adding
more cylinders per coupled wheel set. As locomotives got larger and
more powerful, their reciprocating machinery had to get stronger and
thus heavier, and thus the problems posed by imbalance and hammer blow
became more severe. Speed also played a factor, since the forces
became greater and more destructive at higher wheel speeds. Ralph P.
Johnson thought that the growing size and piston thrusts of existing
express passenger locomotives could not be sustained with the by-then
4-8-4 2-cylinder layout. In addition, he became convinced
that a single pair of cylinders with conventional valve gear and
piston valves was approaching the limits in terms of steam flow.
0-6-6-0T duplex locomotive built by
Jules Petiet in 1863
The earliest attempt at duplex locomotive was an
locomotive designed by
Jules Petiet in 1863 for the French Northern
Railway, but the idea was not perpetuated. However, the innovation of
more rigid hinges that permitted only horizontal swinging movements
and not twisting or vertical movement was from ALCO, and not seen
until 1936's Union Pacific Challenger.
Instead came the idea of having multiple groups of cylinders and
driven wheels mounted in one rigid frame. A "duplex" version of a
4-8-4 would be a 4-4-4-4, with the eight driving wheels split into two
groups of four, each with its own set of cylinders and valve gear. The
reciprocating mass of both sets would be substantially less than the
single set on the 4-8-4, since they would be under less stress and
gentler piston thrusts. The cylinders could be smaller yet have
larger, more efficient valves. The most obvious tradeoff was that in
Baldwin's proposed design, the rigid wheelbase was longer since the
second set of cylinders had to be between the two sets of drivers.
This was sufficiently concerning for many roads, for whom current
locomotives were taxing enough, to reject the duplex idea.
Baltimore and Ohio class N-1 #5600 George H. Emerson
The sole example of the N-1 class.
The first road to use the idea was the Baltimore and Ohio Railroad,
who rejected a Baldwin proposal in 1932–33 but then constructed the
single Baltimore and Ohio Class N-1, #5600 George H. Emerson in the
railroad's own shops without Baldwin's assistance. The locomotive was
completed in May 1937 and managed to retain the same coupled wheelbase
as the road's current 4-8-2s by having the second set of cylinders
reversed, mounted alongside the firebox, and driving the second set of
coupled wheels forwards. This proved to be less than ideal, the size
of the cylinders and firebox both being constrained by this location,
the long steam passages proving problematic, and the cylinders
suffering from the dust and heat of the nearby firebox.
The locomotive was in light service and tested until withdrawal in
1943. It was not successful enough for the B&O to express any
PRR class S1
The S1 at the 1939 New York World's Fair.
The next usage of the duplex type was the Pennsylvania Railroad's
single S1, designed to meet a requirement to operate a 1,200 short
tons (1,070 long tons; 1,090 t) train at 100 mph
(160 km/h) on level track and able to accelerate to that speed
easily. In excess of 4,000 hp (3.0 MW) was necessary for
that requirement, and to meet it Baldwin and the PRR created possibly
the largest passenger steam locomotive ever built, a 6-4-4-6
locomotive 140 feet 2 1⁄2 inches (42.74 m) long
and weighing 1,060,000 lb (480,000 kg) with tender. It was,
in fact, too large to work over the majority of the PRR's system and
was placed into service only between
Chicago, Illinois and Crestline,
Ohio. In service after December 1940, it proved powerful and capable
but prone to wheelslip and surging, presaging the problems with later
duplex designs. By and large, its flaws were written off as simply the
teething troubles of an early, first-cut prototype. It was taken out
of service in 1945. This locomotive got the nickname "The Big Engine"
PRR class T1
T1 prototype PRR 6110 at the
Baldwin Locomotive Works
Baldwin Locomotive Works in 1942.
The S1 did not represent Baldwin's true desires for the type, but in
the design of the T1, of which two prototypes were ordered in July
1940, Baldwin was given much more freedom. The PRR's requirements were
the use of the
Belpaire firebox and the Franklin oscillating-cam
poppet valve gear. The two, #6110 and #6111, were delivered in April
and May 1942. Testing again proved them to be powerful and capable but
T1 number 6110 at Chicago Union Station.
As soon as wartime restrictions on producing passenger locomotives
were eased in February 1945, the PRR placed an order for 50 production
examples. This was a fateful step, since the problems encountered with
the prototypes had not been ironed out nor had they been tested with
the intensity required to be sure of production reliability. The
production locomotives differed in detail in their streamlined
casings, and in the suspension in an attempt to increase adhesion.
Problems became apparent very soon. The locomotives were incredibly
susceptible to violent wheelslip, not just at starting but also at
speed. Dividing the drive into two groups meant that each group was
much more likely to slip. A theory has been advanced that the more
rapid, "sharper" opening of the valves with the poppet valve gear
exacerbated the problem. Worse, such violent high-speed slipping could
damage the valve gear components. This was major problem on a
locomotive with the poppet valve gear because, unlike the familiar
piston valves and outside
Walschaerts valve gear
Walschaerts valve gear of other locomotives,
many of the components were nearly inaccessible within the frame.
Their complexity meant that availability and reliability proved poor,
and while a very capable locomotive engineer (driver) could extract
great performance from a T1, they proved rather unsuccessful in
Whether the problems were soluble has been contentious ever since.
What is undeniable is that the PRR very soon did not care; the much
easier solution of the diesel locomotive was taken within months of
the production locomotives being delivered, and within 2 years all
principal passenger trains were diesel-hauled. The T1s spent a few
more years in lesser service before being withdrawn in the early
The PRR also desired to apply the duplex principle to freight haulage,
and the Q1 was the first experiment in that direction. It was a
4-6-4-4 fast freight locomotive delivered in May 1942. Like the
B&O's George H. Emerson it had the second pair of cylinders facing
backwards, and all were fitted with standard Walschaerts valve gear.
The wheels were of 77 inches (1.956 m) diameter (ill suited for
freight hauling) and the engine was streamlined more like a passenger
engine. The streamlining was later removed as it restricted
maintenance work. The rear facing cylinders were also problematic,
partially due to the inconvenient placement of being directly below
the engine's firebox. Furthermore, the large drivers and smaller than
adequate firebox grate area may have contributed to further problems
as well. The limited firebox size was also a direct effect of the
backwards facing cylinders. The lone Q1, PRR 6130, was constructed at
Altoona Works in 1942.
Production locomotives followed from the end of 1944, but these were
rather different, the lesson that backwards-facing cylinders next to
the firebox were a bad idea having been relearned. The production Q2
locomotives were of
4-4-6-4 arrangement; they were the largest
non-articulated locomotives ever built and the most powerful
locomotives ever static tested, producing 7,987 hp
(5,956 kW) on the PRR's static test plant. The Q2 locomotives
were also the most powerful steam locomotive ever constructed with ten
driving wheels. In operation, the Q2 could outperform pre-existing
freight engines hauling double the tonnage of their predecessors.
Furthermore, the Q2 had no problems building up steam power and was
known to be a very smooth riding engine. Twenty-six of them were built
Altoona Works and they were by far the most successful duplex
type. The duplex propensity to slip was combated by an automatic slip
control mechanism that reduced power to the slipping unit. The slip
control mechanism wasn't always responsive and its complexity often
lead to maintenance crews not wanting to bother with it during
Despite overall success, the Q2s were all out of service by 1951. With
dieselization, they were the obvious first targets to be withdrawn
since they were only a little more capable than the conventional J1
class 2-10-4s, but with far higher running costs and maintenance
A successful French duplex
In France, the duplex type was made famous by the ten 2-4-6-2 (151A)
compound locomotives built in 1932 for the Paris-Lyons-Marseilles
company (P.L.M.) to haul heavy freight trains on the 0.8% grade
between Les Laumes and Dijon. The performance was so good, the company
wanted to order more engines, but the nationalization of the railways
in 1938 stopped all projects. These duplex engines were fitted with
Lenz-Dabeg rotary cam valve motion and soon thereafter with double
exhaust. The low-pressure cylinders drove the first coupled axle, and
the high pressure cylinders the second set of axles. Both groups of
drivers where linked with inside connecting rods through inside cranks
on the 2nd and 3rd drivers, making this locomotive a true 2-10-2, The
driving wheels had a diameter of 1.50 m (4 ft 11 in).
The highest permissible speed was 53 mph (85 km/h). In a
test on December 19, 1933, the engine developed slightly more than
3,000 hp (2,200 kW) at the drawbar over a distance of 37
miles (60 km) and a speed of at least 46 mph (74 km/h),
without being overworked. In ordinary service these engines could haul
1,375 short tons (1,228 long tons; 1,247 t), sustaining
31 mph (50 km/h) at the summit of the 0.8% Blaisy grade.
After electrification of the line, the 151A were sent for service in
northeastern France. They were withdrawn from service in 1956 and
No duplex locomotives survive. Even today, their success and whether
they could have been made to work well given sufficient time and
effort is a controversial subject. A common conclusion is that the
conventional steam locomotive's problems were not as insoluble as
Baldwin believed, and that the duplex arrangement introduced as many
problems as it solved.
A nonprofit group known as the T1 Trust is in the process of
constructing a new duplex locomotive,
Pennsylvania Railroad T1
numbered 5550, intending to utilize design improvements from the
postwar steam era not used or seldom tested on pre-existing T1s in the
hope of creating better performance characteristics. The estimated
year of completion for the project is 2030.
Cravero, Carlo (2011). "Le locomotive a vapore americane tipo
Duplex=American steam locomotives: the Duplex". Ingegneria ferroviaria
(in Italian and English). 66 (5): 413–447.
Reed, Brian (June 1972). Loco Profile 24: Pennsylvania Duplexii.
Staufer, Alvin (1962). Pennsy Power. Staufer. LOC 62-20872.
Vilain, L.M. (1971). L'évolution du matériel moteur et roulant de la
Cie. PLM. Vincent, Fréal et Cie Paris.
^ "FAQ Section - The T1 Trust". The
Pennsylvania Railroad T1 Steam