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Uncoupled Driving Axles and Duplex Locomotives[Inhalt]
Bavarian AA I
German Reichsbahn class 168
Germany | 1896 | only one produced
K.Bay.Sts.B.

The AA I was a prototype used to test ways of saving coal and water by using a lift axle. It was based on the B XI, which was built from 1892 both as a conventional saturated simple locomotive and with a compound engine. The 4-4-0 wheel arrangement of the predecessor was modified by replacing the second coupled axle with a trailing axle. For this purpose, the lift axle with small wheels was used between the lead bogie and the first coupled axle, which was driven by a second engine with two smaller cylinders.

With this configuration, it was possible under most conditions to drive with only one driving axle, thus saving coal and water. The lift axle was raised using a pressure cylinder because running along with the additional engine being turned would have required more power. If more power or traction was needed, the lift axle was lowered and some of the steam was fed to its cylinders. Although the locomotive with this system was economical and reliable on a day-to-day basis, the complicated engine required more maintenance than with conventional steam locomotives. So it was decided to continue to procure the B XI.

After an accident in 1907, the AA I was converted into a 4-4-0 locomotive, making it look like its predecessor. In contrast to this, it received a superheater. With this powerplant, it was able to last the longest of all Bavarian 4-4-0 engines in service with the Reichsbahn and was therefore not retired until 1933 as road number 16 861.

Variantas builtrebuilt P 2/4
General
Built18961907
ManufacturerKrauss
Axle config4-2-2-2 4-4-0 (American) 
Gauge4 ft 8 1/2 in (Standard gauge)
Dimensions and Weights
Length54 ft 4 3/8 in
Wheelbase24 ft 3 5/16 in
Fixed wheelbase8 ft 4 3/8 in8 ft 2 1/16 in
Service weight113,979 lbs
Adhesive weight61,729 lbs65,698 lbs
Axle load32,849 lbs
Water capacity3,830 us gal
Fuel capacity13,228 lbs (coal)
Power
Power sourcesteam
Estimated power671 hp (500 kW)
Optimal speed50 mph26 mph
Top speed56 mph
Starting effort8,587 lbf16,771 lbf
with start valve10,304 lbf
Booster8,085 lbf
Power Plant
Driver diameter73.6 in
Boiler pressure188 psi
Expansion typecompoundsimple
Cylinderstwo, HP: 15 3/16 x 24 in
and LP: 24 x 24 in
two, 17 15/16 x 24 in
Boiler
Grate area24.3 sq ft
Firebox area102.3 sq ft96.9 sq ft
Tube heating area1,257 sq ft1,018.3 sq ft
Evaporative heating area1,359.3 sq ft1,115.1 sq ft
Total heating area1,359.3 sq ft1,115.1 sq ft
Calculated Values
steam locomotive
express
passenger
lift axle
booster
prototype
last changed: 01/2022
French Western Railway No. 500
France | 1884 | only one produced

After the compound locomotives of the Webb type were apparently successful in Great Britain, the Ouest also ordered one locomotive for trials. Like the British locomotives, it had two small outer high-pressure cylinders driving the second driving axle and one large inner low-pressure cylinder driving the first driving axle. These two axles were not coupled to each other. The number 500 was rebuilt in the meantime, but was already retired in 1901.

General
Built1884
ManufacturerSharp, Stewart & Co.
Axle config2-2-2-0 (Webb Compound) 
Gauge4 ft 8 1/2 in (Standard gauge)
Dimensions and Weights
Fixed wheelbase8 ft 3 1/16 in
Service weight81,571 lbs
Adhesive weight57,541 lbs
Axle load29,542 lbs
Fuel capacitycoal
Power
Power sourcesteam
Estimated power469 hp (350 kW)
Optimal speed33 mph
Starting effort8,976 lbf
with start valve10,771 lbf
Power Plant
Driver diameter78 in
Boiler pressure152 psi
Expansion typecompound
Cylindersthree, HP: 13 x 24 in
and LP: 26 x 24 in
Boiler
Grate area16.8 sq ft
Firebox area100.1 sq ft
Tube heating area963.4 sq ft
Evaporative heating area1,063.5 sq ft
Total heating area1,063.5 sq ft
Calculated Values
steam locomotive
last changed: 04/2023
Chicago, Milwaukee, St. Paul & Pacific (Milwaukee Road) “James Toleman”
United States | 1893 | only one produced
The locomotive before changed to American standards
The locomotive before changed to American standards
Railway and Locomotive Engineering, October 1898

The “James Toleman” was developed by Frederick Charles Winby to test the installation of a long firebox between uncoupled axles. Each axle had its own power plant, with the inner cylinders driving the first driving axle. Thus one could do without long coupling rods. The boiler was oval in cross-section to fit between it large wheels. After attending the Columbian Exposition, she was given to the Milwaukee Road for trials. However, it quickly became apparent that she repeatedly had too little steam and parts regularly broke. Even a lengthy visit by Winby to the USA could not solve the problems that had arisen.

General
Built1893
ManufacturerHawthorn, Leslie & Co.
Axle config4-2-2-0 
Gauge4 ft 8 1/2 in (Standard gauge)
Dimensions and Weights
Service weight120,000 lbs
Adhesive weight70,000 lbs
Axle load35,000 lbs
Power
Power Plant
Driver diameter90 in
Boiler pressure175 psi
Cylindersfour, front: 17 x 22 in
rear: 16 1/2 x 24 in
Boiler
Grate area28 sq ft
Firebox area182.6 sq ft
Tube heating area1,817.4 sq ft
Evaporative heating area2,000 sq ft
Total heating area2,000 sq ft
Calculated Values
steam locomotive
express
prototype
duplex
last changed: 07/2023
Pennsylvania class S1
United States | 1939 | only one produced
S1 in November 1942 with the “Trail Blazer” from New York to Chicago
S1 in November 1942 with the “Trail Blazer” from New York to Chicago
PRR1944 / PRR S1 6100 hauling the Trail Blazer in Nov 1942

In the search for a very powerful express locomotive to replace the K4 Pacifics, the PRR stumbled upon the problem of the excessive moving masses. The seven feet driving wheels would have resulted in extremely long coupling rods in a conventional, eight-coupled locomotive. These would not only have transmitted a strong hammer blow to the rails and locomotive, but could also have broken easily. The solution was the design of the duplex locomotives, which combined two separate steam engines on a common frame. To carry the heavy weight of the locomotive, a three-axle bogie was needed in front of and behind the coupled axles. This allowed less than half the locomotive's weight to be used as adhesive weight, further increasing the tendency to slip.

Star designer Raymond Loewy provided the locomotive with streamlined Art Deco-style cladding and, including the eight-axle tender, reached a total length of more than 140 feet. Since they no longer fitted on the turntables, the PRR built special track triangles for turning. Despite the bogies that could be moved laterally by 2.25 inches, it soon became clear that the S1 had a tendency to derail not only in many curves, but also in the wyes specially built for them. As a result, they could not be used on the entire route between Chicago and Pittsburgh as planned, but only on two-thirds of this route. In addition, if the coupling wheels slipped, it was almost impossible for the engine driver to determine which of the two engines was spinning and had to be braked accordingly. These were the main reasons why series production only came about with the T1 with smaller wheels and two-axle bogies

With regard to the performance of the locomotive, there are different information, since it was never run at the highest speeds together with dynamometer cars. What is undisputed is that they could pull the required 1,350-ton trains at 100 mph, giving an output of 7,200 hp. Higher speeds were often made to compensate for delays, although there were different observations regarding the maximum speed. Presumably more than 125 mph were reached several times, but there are also indications in the range between 135 and 145 mph. Allegedly, the PRR once paid a fine when the locomotive reached 156 mph km/h. Irrespective of this, the one-off, which could not be operated economically, was decommissioned in 1946 and scrapped in 1949.

General
Built1939
ManufacturerAltoona
Axle config6-4-4-6 (Duplex) 
Gauge4 ft 8 1/2 in (Standard gauge)
Dimensions and Weights
Length140 ft 2 1/2 in
Wheelbase64 ft 4 in
Fixed wheelbase26 ft 6 in
Total wheelbase107 ft 7 1/2 in
Service weight608,170 lbs
Adhesive weight281,440 lbs
Total weight1,060,010 lbs
Axle load74,000 lbs
Water capacity24,230 us gal
Fuel capacity52,900 lbs (coal)
Power
Power sourcesteam
Indicated power7,200 hp (5,369 kW)
Optimal speed60 mph
Top speed120 mph
Starting effort76,403 lbf
Power Plant
Driver diameter84 in
Boiler pressure300 psi
Expansion typesimple
Cylindersfour, 22 x 26 in
Boiler
Grate area132 sq ft
Firebox area660 sq ft
Tube heating area5,001 sq ft
Evaporative heating area5,661 sq ft
Superheater area2,085 sq ft
Total heating area7,746 sq ft
Calculated Values
steam locomotive
express
duplex
streamline
last changed: 03/2022
Pennsylvania class T1
United States | 1942 | 52 produced
The first prototype No. 6110
The first prototype No. 6110
Wilkes-Barre Record

After the S1 built at the Altoona Works proved too large and of little practical use, Baldwin built the first two prototypes of the T1 for the PRR during World War II. The reason for purchasing such large express locomotives was that the trains had become heavier and heavier and the K4 class Pacifics used often had to be used double-headed. Like the S1, the T1 was of the duplex type and was streamlined by designer Raymon Loewy.

The two separate engine groups in one frame were necessary because large moving masses were created with a large wheel diameter and four coupled axles. These stressed the crew of the locomotive and exerted large impacts on the track. For this reason, conventional three- and four-cylinder engines were used in large express locomotives in other countries, which were unpopular in the USA due to the poor accessibility of the inner cylinders.

No. 5507 in December 1945 with a passenger train exiting Chicago
No. 5507 in December 1945 with a passenger train exiting Chicago
collection Taylor Rush

The special feature of the T1 was that two two-cylinder engines, each with two coupled axles, were located one behind the other in a common frame. The biggest disadvantage of the duplex engines was the increased tendency to slip. If one engine group started to slip, the crew could not tell which axles were slip with the help of the exhaust blasts, and they had to be very sensitive with the throttle, especially with such powerful locomotives. For this reason, the rear bogie of the second prototype was designed as a switchable booster to increase the adhesive weight when starting.

When the construction of passenger steam locomotives was possible again after the end of the Second World War, the PRR ordered 50 production machines without boosters. They were significantly more suitable for everyday use than the S1 and still achieved an output of more than 6,000 hp. This enabled them to pull trains weighing around 1,000 tons at 100 mph without any problems and is said to have reached a regular speed of 120 mph to make up for delays. As with the T1, there are unconfirmed reports of trips in excess of 125 mph. The PRR's decision in 1948 to convert all high-value passenger trains to diesel had a negative impact on the service life of the T1. This was also be implemented in a timely manner, so that all T1s were already retired between 1949 and 1952 and subsequently scrapped.

The organization “The T1 Trust” was founded with the aim of building a working T1 from the ground up. Work began in 2014 and by 2020 large parts of the boiler, the driver's cab, parts of the fairing and two coupled wheels had already been completed. An eight-axle model was bought as a tender, which comes from an M1 and is to be provided with a streamlined fairing. After the planned completion in 2030, the locomotive should be used regularly and help to reconstruct the true performance of the originals. With a targeted maximum speed of 140 mph, the previous speed records for steam locomotives are to be broken. In January 2022, the organization said it was 39.1 percent complete on the loco.

General
Built1942, 1946
ManufacturerAltoona, Baldwin
Axle config4-4-4-4 (Duplex) 
Gauge4 ft 8 1/2 in (Standard gauge)
Dimensions and Weights
Length122 ft 9 3/4 in
Wheelbase51 ft 11 in
Fixed wheelbase13 ft 10 in
Total wheelbase107 ft
Service weight502,200 lbs
Adhesive weight279,910 lbs
Total weight944,700 lbs
Axle load71,680 lbs
Water capacity19,200 us gal
Fuel capacity85,200 lbs (coal)
Power
Power sourcesteam
Indicated power6,500 hp (4,847 kW)
Optimal speed64 mph
Top speed110 mph
Starting effort64,653 lbf
Power Plant
Driver diameter80 in
Boiler pressure300 psi
Expansion typesimple
Cylindersfour, 19 3/4 x 26 in
Boiler
Grate area92 sq ft
Firebox area490 sq ft
Tube heating area4,209 sq ft
Evaporative heating area4,699 sq ft
Superheater area1,430 sq ft
Total heating area6,129 sq ft
Calculated Values
steam locomotive
express
duplex
streamline
last changed: 01/2022
Pennsylvania class Q2
United States | 1944 | 26 produced
Nr. 6180 at Crestline, Ohio
Nr. 6180 at Crestline, Ohio
collection Taylor Rush

The Q2 was created after the duplex prototype of the Q1 class with three driving axles at the front and two at the rear. This was only intended for freight transport and the number of driving axles in the first and second chassis groups was reversed compared to the Q1. As with the Q1, all the driving axles were mounted in the frame and the engines were divided to avoid too long connecting and coupling rods

Since the front chassis group had only two axles, the cylinders here were smaller than in the rear. In contrast to the Q1, the rear cylinders were still in front of the associated driving axles. Each group was prevented from slipping by an automatic apparatus. A booster was also installed in one axle of the trailing bogie

Overall, the Q2 is considered the largest non-articulated steam locomotive of all time. In static testing, one of the locomotives achieved an output of almost 8,000 hp. This made it possible to pull 125 loaded freight cars at a speed of 50 mph. On the other hand, maintenance was very complex.

In addition to the prototype, 25 production locomotives were built, making the Q2 the most successful duplex locomotive. However, the PRR fleet also included the class J1 with the 2-10-4 wheel arrangement which, with a nominal output of 5,650 hp, achieved almost the same starting tractive effort and was significantly easier to maintain. In addition, more and more diesel locomotives were added. This resulted in the Q2 being out of service by 1951.

General
Built1944-1945
ManufacturerAltoona
Axle config4-4-6-4 (Duplex) 
Gauge4 ft 8 1/2 in (Standard gauge)
Dimensions and Weights
Length124 ft 7 1/8 in
Wheelbase53 ft 5 1/2 in
Fixed wheelbase20 ft 4 in
Total wheelbase107 ft 7 1/2 in
Service weight619,100 lbs
Adhesive weight393,000 lbs
Total weight1,049,100 lbs
Axle load79,780 lbs
Water capacity19,000 us gal
Fuel capacity75,000 lbs (coal)
Power
Power sourcesteam
Indicated power7,987 hp (5,956 kW)
Optimal speed63 mph
Starting effort100,816 lbf
Booster15,000 lbf
Power Plant
Driver diameter69 in
Boiler pressure300 psi
Expansion typesimple
Cylindersfour, front: 19 3/4 x 28 in
rear: 23 3/4 x 29 in
Boiler
Grate area121.7 sq ft
Firebox area725 sq ft
Tube heating area6,000 sq ft
Evaporative heating area6,725 sq ft
Superheater area2,930 sq ft
Total heating area9,655 sq ft
Calculated Values
steam locomotive
freight
duplex
booster
last changed: 07/2023
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